I have the exact same issue with a PR for my half marathon. My 645 measured 21,25km and took the accompanied time, instead of the time for 21,10km. Strava took the time for my PR at 21,10km.
I'm sure that you didn't run more than 21,1km. There are several things that affect GPS accuracy. I ran HM that my watch measured shorter. One time it was set as a HM record, the other time I told GC that it was a HM with new PB. So what if your watch would measure a shorter distance and adds the difference to your time, so that it isn't your PB anymore?