As soon as i got world adventures i started a new sim, athletic, lucky, brave and disciplined, with martial arts, trained my lvl 10 athletics sim to the max, broke countless space rocks.... then he was beaten 8 times in a row by his teen sister that had no athletics, was a couch potato and only had lvl 1 martial arts.
I later found that this wasn't just bad luck due to the sadorandom number generator, this was an incredibly common occurrence; it didn't mattered that i had 783 wins under the belt, J.R. Nobody with lvl 1 martial arts and all the possible bad moodlets and bad traits still had decent chances to beat my guy.
A check to MATY forums and pescado confirms it, the equation used to check this is very similar in form to the one that caused this:
Ladies and gentlemen, we have a new spearman beats tank.