Hi,
I feel the same, I know it is an approximate and all but I had the Fenix 3 HR for a year prior to the Fenix 5 and my VO2max would increase and decrease with my training, not by much but surely it did, not just my training but how i felt, after a race I had last year I stopped training it surely went down from 44 to 42, it slowly picked up when I started training again, not just the number but how I felt while running.
It was at 44 when I changed to the F5 and bam! it went down instantly without me decreasing my training, I even had a race I was training for with it and it kept on going down, now it stays at 39 and won't move even though I feel I run better (and had a much better race this year).
Maybe one is wrong and the other is right or both are wrong, but how come those two watches from the same series have such different results? and why the F3HR actually reflected at least in proportion to the way I feel?
Anyways, I'm male, 36 years old, not super fast, usually run steadily at 6:30 min/km.