My guess would be that there's a hidden sparring skill, just like the hidden chess skill, and you need to spar a lot to get better at it.
They must be doing an excellent job at keeping this hidden. Because they sure are leaving out all traces and signs of this being the case. My sim had max everything as well. And believe me, my sim has sparred plenty. I don't know what qualifies as "Alot" But making all the needs static, and proceeding to have my sim do one match after another, constantly for about 2 sim weeks appeared to do very little. After the two weeks, and after my sim at the end losing 6-7 matches in a row... I gave up on the whole idea.
Until someone can show otherwise, I've made the conclusion it's purely a "Flip of a coin" type of thing. 50/50. Disciplined trait increasing chances? Bah. Must be a myth. An excuse for EA to add one more awkward/out of place idle animation. And actual skill levels? Nah those can't be affecting anything other than what colors your sim decides to wear while trainin'. It's lies! All lies!