That's a scoring system also used in games of induction (well, at least, those organised enough to have a scoring system, i.e. not Mao). Typically each guessing player scores based on how efficiently they guess the rule, and the rule-setter scores in some way that's based on the spread of the guessers' scores, so that (as you say) they're encouraged to set problems at a difficulty level that differentiates players from each other. Of course setters' and guessers' scores aren't directly comparable, so everyone has to take a turn at setting before you can sensibly work out who won.