VO2 max Test

tefkar said:

The watch seems to be getting this from my normal runs. 

You're correct.

A company called FirstBeat developed the algorithm (recently actually purchased by Garmin). Basically, it uses your Heart Rate compared to your pace along with your weight.  So how much apparent work are you doing to get a specific outcome.  Lots of other things involved and needs accurate max heart rate. Works best running a decent pace steady on a flat track (I think it needs about 1km minimum). as soon as you add hills and different terrain the result may be less useful. It is only recently that you can enable it with the trail run activity but still will have issues with tough terrain (algorithm “sees” a lot of effort but a reduced pace). 
Similar on the bike where you need power from a meter (similar to pace) to compare to the HR. As for most algorithms, it works well for some and less well for others. When I was doing a lot of road work my Garmin VO2Max was fairly close to a full treadmill test. I ignore it now as most of my running is trail and Orienteering (and snowshoe running in winter), non of which are handled by the algorithm. 

Just to stress again, this is a simplistic explanation. Also, as you may be seeing, it takes a number of runs to start getting “accurate”.  The watch test you can do is just a formal version of the same thing. Nothing beats a proper lab test. 

Here's a relevant article for you:

https://www.firstbeat.com/en/science-and-physiology/fitness-level/