The original poster is quite correct. My sim fu king grand master is having the same problem. He has to fight every opponent sometimes two or three times in order to win a single match whether it's just sparing or a tournament match.
His athletics is at level 10 and he's completed the body builder challenge.
His martial arts level is at level 10 and he's completed all challenges in that skill.
His chess/logic skill is at level 7.
I took the liberty of looking up the last sparing opponents stats. They were as follows. Tournament experience was 2 wins, 1 loss. Athletic was level 9. Martial Arts 10. Chess/Logic level 4. Something isn't adding up here at all. I wouldn't mind one or two losses every now and then against an evenly matched Sim but to lose almost constantly kind of negates the whole "Guaranteed you'll rarely lose" point of obtaining the Sim Fu King challenge.
It is rather annoying when NPC sims with practically no fighting experience still beat sims all the time with a significant amount of experience and much greater skill levels etc.
Edit: Forgot to Add my Sim is at a Level 5 tournament rank.