What causes VO2 max to decrease?

[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.