Try to better explain why

See https://github.com/element-hq/synapse/pull/17293#discussion_r1633904606

synapse/storage/databases/main/stream.py

@@ -914,11 +914,17 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
         def get_last_event_in_room_before_stream_ordering_txn(
             txn: LoggingTransaction,
         ) -> Optional[str]:
-            # We need to handle the fact that the stream tokens can be vector
-            # clocks. We do this by getting all rows between the minimum and
-            # maximum stream ordering in the token, plus one row less than the
-            # minimum stream ordering. We then filter the results against the
-            # token and return the first row that matches.
+            # We're looking for the closest event at or before the token. We need to
+            # handle the fact that the stream token can be a vector clock (with an
+            # `instance_map`) and events can be persisted on different instances
+            # (sharded event persisters). The first subquery handles the events that
+            # would be within the vector clock and gets all rows between the minimum and
+            # maximum stream ordering in the token which need to be filtered against the
+            # `instance_map`. The second subquery handles the "before" case and finds a
+            # row before the token. We then filter out any results past the token's
+            # vector clock and return the first row that matches.
+            min_stream = end_token.stream
+            max_stream = end_token.get_max_stream_pos()
 
             # We use `union all` because we don't need any of the deduplication logic
             # (`union` is really a union + distinct). `UNION ALL`` does preserve the
@@ -956,10 +962,10 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
                 sql,
                 (
                     room_id,
-                    end_token.stream,
-                    end_token.get_max_stream_pos(),
+                    min_stream,
+                    max_stream,
                     room_id,
-                    end_token.stream,
+                    min_stream,
                 ),
             )