Dumb question - did you sync your watch with Garmin connect just before comparing the two? Race prediction times can definitely change after a workout, even if your VO2 max or lactate threshold don't change. I'm not sure if race prediction times can change as a result of tracking done by the watch in smartwatch mode (e.g. your HR zones change as a result of RHR changing).
This error is just a side effect of other infamous bug: LHTR not being synced into Garmin connect. So you get different predictions on your watch than in connect.
Maybe because on the watch is shown round figures. In my watch my run today is 48 vo2. In the file in Runalyze it shows as 47,69. That could explain if the race predictor uses an othe parameter than the vo2 max. It isn’t that far apart.
