Why? The only favourable circumstance I can think of is that Kenobi knew Anakins style, but that's true the other way around as well.
The entire match Anakin was blinded by his rage and not thinking straight or about the enviroment at all. Kenobi on the other hand was constantly trying to gain a terrain advantage, which he eventually succeeded in. Thus favorable circumstance.
If you not right before the end, when Obi-wan is saying "i have failed you anakin, I have failed you" and Anakin says "this is the end for you my master" that Obi-wan is clearly the more tired of the 2. Had it been on even terrain that was never shifting, Obi-wan never would have won.