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.
Those are not favourable circumstances, those are character strengths and weaknesses. Anakin's rage is his own shortcoming, and Obi-Wan's excellent use of terrain is a proof of his tactical superiority. It's like saying Anakin beat Cin Drallig because of favourable circumstances, which circumstances are he is a better duelist and stronger in the force