Good question.
Garmin has been lowering my VO2 max over the past 6 weeks and my FTP went up.
They have it all wrong if you ask me.
No 64 year old man in the world has a VO2 max of 73 and lowering it to 61 while FTP is increasing. That ain't wonky, it is wacky wrong. I think I know the source of their error but really don't want to get that deep on this .