How is VO2 Max measured and recorded for the Fitness app on iPhone

The fitness app in iPhone generates a trend line for VO2 Max. How is this being recorded? I always record my workouts - cycling / walking / weight training - and none of those individual results reports VO2 Max.

I know it is not fully accurate, but represents a trend line, however, over the past 12 months I have gone from an extensive period of very little activity post-surgery, to a regular regime of cardio rehab. I have two VO2 Max readings from fitness tests and the numbers are quite different from those reported in the fitness app.

[Re-Titled by Moderator]