Here’s an example:
I got this email the other day, about my percentage of running Beacon Hill, which apparently is in Boston. The last time I ran there was in Dec 2023.
Me getting this email feels like a bug.
Those can be sent if the city is updated in a way that adjusts your percentage.
You shouldn’t get multiple emails for the same percentage completion, though.
This may have been my “fault”. I recently made some OSM edits in Beacon Hill, and I think the net number of streets may have been reduced as a result (and hence your percentage went up). 
Thanks for the replies and explanation. Feels weird to get notifications about changes in percentage about places where have run in the past but I am not tracking for City Strides. In City Strides I set the cities that I want to be notified about changes, and Boston is not among the two cities I selected.
Percentage progress updates and city update notifications are different. In your Notification settings, you can toggle “City Progress” for the milestone notifications and you can toggle specific cities in the “City Updates” section for notifications when a city gets updated from OSM.
Thank you, I turned off City Progress.
I got a city completion email once during an update, even though after the update I had not in fact completed the city (was a few streets short).
My guess is that my progress hit 100% due to deletions, but before updates/creations were accounted for.
Yeah, that’s an edge case in the sync code