Controlling when VO2 Max is Measured

So, over the last two years, I have done a lot of hard work to get into better shape. Last year, I bought an Apple Watch 4, and over the course of several months, have been tracking my estimated VO2 max values. However, I have an issue with when my Apple Watch and iPhone decide to measure my VO2 max. A lot of days, I will take a brisk (though hardly strenuous) walk after lunch. Then, after work, I will go to the gym and do a vigorous 60 minute workout on an elliptical. It only seems to measure my VO2 max during the light walk, and not during the heavier workout, which it seems would yield a higher VO2 max number. And there doesn't seem to be a way to kick off an ad hoc measurement of VO2 max. So, all of my numbers appear to be fairly mediocre - not bad, but not what I would hope for given the amount of work I have put into getting into shape. I feel like the mediocre numbers are a result of when VO2 max is measured - during a light workout period.

Any insights would be appreciated.