I’m using the garmin connectIQ plug ins from xert. On harder rides, XSS reads about 5-10% higher during a ride than in post workout analysis. During todays ride, Garmin was showing a breakthru for about a minute, while post workout showed that MPA was about 100-300 watts higher during this portion.
If you had a BT, go to advanced MPA and click previous (signature). You will see that XSS will differ, i.e. will be higher against your old signature. That view will also show more accurately the exact moment you drew MPA below TP and show an effort above MPA (yellow line).