diff --git a/synapse/handlers/stats.py b/synapse/handlers/stats.py
index 8e1bf8b5d5..7536e1a54c 100644
--- a/synapse/handlers/stats.py
+++ b/synapse/handlers/stats.py
@@ -76,20 +76,18 @@ class StatsHandler(StateDeltasHandler):
             # we only want to run this process one-at-a-time,
             # and also, if the initial background updater wants us to keep out,
             # we should respect that.
-            try:
-                run_as_background_process("stats.notify_new_event", process)
-            except:  # noqa: E722 – re-raised so fine
-                lock.release()
-                raise
+            run_as_background_process("stats.notify_new_event", process)
 
     @defer.inlineCallbacks
     def _unsafe_process(self):
         # If self.pos is None then means we haven't fetched it from DB
+        # If None is one of the values, then means that the stats regenerator has not (or had not) yet unwedged us
+        #   but note that this might be outdated, so we retrieve the positions again.
         if self.pos is None or None in self.pos.values():
             self.pos = yield self.store.get_stats_positions()
 
-        # If still None then the initial background update hasn't started yet
-        if self.pos is None or None in self.pos.values():
+        # If still contains a None position, then the stats regenerator hasn't started yet
+        if None in self.pos.values():
             return None
 
         # Loop round handling deltas until we're up to date
@@ -141,7 +139,10 @@ class StatsHandler(StateDeltasHandler):
                 continue
 
             if event_id is None and prev_event_id is None:
-                # Errr...
+                logger.error(
+                    "event ID is None and so is the previous event ID. stream_id: %s",
+                    stream_id,
+                )
                 continue
 
             event_content = {}
@@ -153,11 +154,14 @@ class StatsHandler(StateDeltasHandler):
 
             # We use stream_pos here rather than fetch by event_id as event_id
             # may be None
-            now = yield self.store.get_received_ts_by_stream_pos(stream_pos)
-            now = int(now)
+            stream_timestamp = yield self.store.get_received_ts_by_stream_pos(
+                stream_pos
+            )
+            stream_timestamp = int(stream_timestamp)
 
+            # All the values in this dict are deltas (RELATIVE changes)
             room_stats_delta = {}
-            room_stats_complete = False
+            is_newly_created = False
 
             if prev_event_id is None:
                 # this state event doesn't overwrite another,
@@ -198,9 +202,9 @@ class StatsHandler(StateDeltasHandler):
                 elif prev_membership == Membership.BAN:
                     room_stats_delta["banned_members"] = -1
                 else:
-                    err = "%s is not a valid prev_membership" % (repr(prev_membership),)
-                    logger.error(err)
-                    raise ValueError(err)
+                    raise ValueError(
+                        "%r is not a valid prev_membership" % (prev_membership,)
+                    )
 
                 if membership == prev_membership:
                     pass  # noop
@@ -213,9 +217,7 @@ class StatsHandler(StateDeltasHandler):
                 elif membership == Membership.BAN:
                     room_stats_delta["banned_members"] = +1
                 else:
-                    err = "%s is not a valid membership" % (repr(membership),)
-                    logger.error(err)
-                    raise ValueError(err)
+                    raise ValueError("%r is not a valid membership" % (membership,))
 
                 user_id = state_key
                 if self.is_mine_id(user_id):
@@ -232,7 +234,7 @@ class StatsHandler(StateDeltasHandler):
                         delta = +1 if membership == Membership.JOIN else -1
 
                         yield self.store.update_stats_delta(
-                            now, "user", user_id, {field: delta}
+                            stream_timestamp, "user", user_id, {field: delta}
                         )
 
             elif typ == EventTypes.Create:
@@ -250,7 +252,7 @@ class StatsHandler(StateDeltasHandler):
                     },
                 )
 
-                room_stats_complete = True
+                is_newly_created = True
 
             elif typ == EventTypes.JoinRules:
                 old_room_state = yield self.store.get_room_state(room_id)
@@ -269,7 +271,9 @@ class StatsHandler(StateDeltasHandler):
                         prev_event_id, event_id, "join_rule", JoinRules.PUBLIC
                     )
                     if is_public is not None:
-                        yield self.update_public_room_stats(now, room_id, is_public)
+                        yield self.update_public_room_stats(
+                            stream_timestamp, room_id, is_public
+                        )
 
             elif typ == EventTypes.RoomHistoryVisibility:
                 old_room_state = yield self.store.get_room_state(room_id)
@@ -289,7 +293,9 @@ class StatsHandler(StateDeltasHandler):
                         prev_event_id, event_id, "history_visibility", "world_readable"
                     )
                     if is_public is not None:
-                        yield self.update_public_room_stats(now, room_id, is_public)
+                        yield self.update_public_room_stats(
+                            stream_timestamp, room_id, is_public
+                        )
 
             elif typ == EventTypes.Encryption:
                 yield self.store.update_room_state(
@@ -312,9 +318,9 @@ class StatsHandler(StateDeltasHandler):
                     room_id, {"canonical_alias": event_content.get("alias")}
                 )
 
-            if room_stats_complete:
+            if is_newly_created:
                 yield self.store.update_stats_delta(
-                    now,
+                    stream_timestamp,
                     "room",
                     room_id,
                     room_stats_delta,
@@ -323,7 +329,7 @@ class StatsHandler(StateDeltasHandler):
 
             elif len(room_stats_delta) > 0:
                 yield self.store.update_stats_delta(
-                    now, "room", room_id, room_stats_delta
+                    stream_timestamp, "room", room_id, room_stats_delta
                 )
 
     @defer.inlineCallbacks