I had my HRmax and heart rate zones entered in Gramin Connect after my last endurance test. Everything was synchronised correctly to my FR 920XT. But turned out that I had HRmax auto-detection turned on and during one of the training sessions I happened to score a higher HRmax than the one I entered and synchronised.
Since HR zones are kept as %HRmax, the HR zone showed during the training and the time spent in zones summary were different to what I was expecting according to what I could see in raw HR data (on the plot).
Basically the problem is that the detected HRmax was not synchronised back to Garmin Connect at which point I'd expect it to be for example marked as "detected" (vs. entered manually), so I could figure out what's going on.