I believe the developpers didn't have time to correct all the issues pointed out by the testers. I've read somewhere vague complaints about this problem (vague because of non-disclosure agreements, etc.).
And personally, I think that horsemen have been overnerfed. They're better balanced than at release, but seriously, who builds horsemen anymore...
The 4 movement of Horseman are great, they are the fastest units until Lancers. The move after attacking works weel with that movement. So a strength of 10 is good as balance. I don't prioritize mounted units, because the movement is not my biggest concern, but when I use them it is really great.