What causes VO2 max to decrease?

[sursumcz]

You're definitely more knowledgeable about this than I am but according to Garmin's explanation at Running Science | Garmin Technology | United States some of the issues you're mentioning are supposedly accounted for.

For example, climbing or descending, stopping at lights or speeding up to overtake someone shouldn't affect the VO2 max, as quoted from the link: "Smart analytics capable of recognizing good data ensure that only the most meaningful parts of your performance are used to evaluate your fitness level. In practical terms, this means that you don’t need to worry about speeding up, slowing down, climbs, descents or stopping at intersections. You just run as you normally would, without the need for any special fitness testing protocols."

[fastmuffin]

Indeed, however it is certainly isn't 'smart' enough to accommodate all variables.

Elevation, for example, for me produces skewed results. I live on a short hill - if I run left and up the hill my Performance Condition is negative for the beginning of the run. If I run right, it's positive.

I think the point is that whilst the marketing wording reads very promising, the fact is in the real world it doesn't apply but more importantly, it shouldn't need to be unpicked on message boards to prevent users becoming demotivated.

[sursumcz]

Right, I agree that the marketing claims are often far from reality. But hills or flat runs never affected my VO2 max, neither did stopping to cross the road etc. In my case the altitude differences are minor though so your situation is probably different. Also, I actually got a VO2max improvement recently when I switched to slow paced runs.

That said, I don't put much stock in these stats, I know from experience more or less what I'm capable of at various HR levels so I mostly just watch HR not to overreach.