I had a "below average" VO2 max score based on my Apple Watch measurements. It was ~40 mL/kg/min, in the span of about a month it jumped up to 53 mL/kg/min, which is "high" for my age group. So what happened? I started running instead of cycling as my primary form of cardio.

My hypothesis is that the apple watch estimates higher if you are running rather than pedaling. I definitely don't think my cardio vascular went from poor to great over a month. It seems more likely that it was maybe underestimating, and perhaps now is overestimating.