Cavalry is mobile on the battlefield, not in travel. Funny fact: Humans are faster at overland travel than horses. While horses have a higher maximum speed, they can't maintain it for long and need more rest. With armies you also have the whole baggage train thing - you need to move more than just the soldiers.
I actually believe that cavalry is simulated a lot more realistically in BM than in most other games.
Yes, but the mobility on the battlefield was what I meant, whether or not you actually end up fighting on a given battlefield. Given that we can't simulate many of the other benefits of cavalry, like flanking maneuvers and hunting down broken forces and the like, I think having them handle evasion differently seems to make sense.