I've noticed a few errors in my watch since having it 8 weeks ago that all seem to step from similar things. Since getting the watch I've done about 1,000 km with a multitude of workouts with intervals lasting from minutes to hours so it should have good data but it doesn't show it. My HR max, min and zones seem fine and my weight is current.
The race predictor is slow to update. I've beaten the predictions many times from 5, 10, and half (haven't run marathons in training) and yet the times predicted by garmin are slower by many minutes - just doesn't make sense.
Related the Vo2 max has stayed the same the whole time despite training faster, lowering the weight number etc.
I've had "maintaining" since July 22. That was after literally 1 day of productive and 3 weeks of unproductive or over reaching. My fitness has greatly improved - you can see my HR on all runs has gone down etc.
Weather - okay it's a bit pedantic that you only get weather acclimatization if your run is over 22c. Although I swear on a 22c day they still give me nothing (I just don't loose it as fast). However this is honestly silly. You should be able to set your own range to see what you're adapting to. Anything 17c and above feels the same where I am where the humidity is 90% or more daily. Since humidity is not factored in, daily runs of 21c with 90% humidity doesn't count, and in fact will drop the index greatly day to day. I'm sure that also has impacts on the training results.
Doubles - if you run a double and your HR is higher garmin doesn't really understand the concept of running a bit tired. I'm probably losing fitness as I get -3 when it beeps in at 6min.
