There appears to be no reasonable explanation why this Middlesex event should find itself added to a Gloucestershire collection. FamilySearch records seem to be indexed far too often against the wrong English county - this is just one example.
@Paul W I've been using the Rhode Island, USA collections quite a bit over the last few days because one of my County Tyrone, Ireland, families settled in Rhode Island. Numerous Rhode Island marriage records are in the Rhode Island Town Deaths Index, 1639-1932. It's a bit unsettling to see marriages labeled as deaths. Here's one example of many: https://www.familysearch.org/ark:/61903/1:1:Q24H-7GH5
I've been pondering where/how to report the issue.