diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index 1422843af8..4ced3144c8 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -1553,7 +1553,8 @@ def _calculate_state(
     # as we may not have sent them to the client before.  We find these membership
     # events by filtering them out of timeline_start, which has already been filtered
     # to only include membership events for the senders in the timeline.
-    # In practice, we can do this by removing them from the p_ids list.
+    # In practice, we can do this by removing them from the p_ids list,
+    # which is the list of relevant state we know we have already sent to the client.
     # see https://github.com/matrix-org/synapse/pull/2970
     #            /files/efcdacad7d1b7f52f879179701c7e0d9b763511f#r204732809