Cycling Fitness / Training Advice

I actually feel fine. But last Nights VO2 intervals (5min Climbs) were like 20w Down on expected.

Then my Garmin said congrats. New VO2 max detected!

Really Garmin?

At a guess:

Lower HR due to fatigue. Lower average HR and the Garmin thinks you've got more spare capacity and so ups the expectations but when you're that fatigued something else gives out (legs, lungs, head) before you can get your HR up where it usually is let alone sustain it there.