My sense is that by telling MFP your "general" activity level during setup, it determines a fixed daily calorie burn rate for you. In my case, that number is 2220 calories (click on Home -> Goals to see your values).
What happens next? On a given day I go for a run, lift some weights, etc. and thankfully the corresponding calories burned during those activities show up automagically on MFP (details are under the Exercise tab). So far, so good. Today, for example, those exercise calories totaled 1247. What MFP *should* do is simply take the total number of calories burned reported by the Band and log them but, but it doesn't. Instead it says, "Well, we've determined that you were supposed to burn 2200 calories today -- so now lets add the 1247 you did from exercise, for a total of 3447. The band is currently saying you've burned a total (including exercise) of 2921, which is less than our estimate of 3447. Therefore we're sticking with our estimate, and your Microsoft Health Calorie Adjustment is 0."
In other words, it's only when the total calories burned reported by the Band exceeds what MFP estimates you should have burned, that it then allows those "extra" calories as a positive adjustment. In your case, those are presumably the days where you had adjustments of 419 and 466. On the other days, your Band reported values must have fallen under MFP's estimate.
If my logic is right, this is all very silly, and premised on the faulty assumption that you'll burn the exact same number of calories each day. I understand the idea of setting a fitness target but this is dumb. As I've ranted on another thread, MFP should simply say, "Your Band says you burned 2921 calories today. Congratulations!" Anything else is clumsy and confusing. :angry:
-Matt