hey you know how sans determines how many times you've fought him based on facial expression? he can tell when you've been dunked on, when you see his first attack for the first time, when you've survived it once, when you've survived it twice... all based on facial expression. each time you die to him and reset, he has absolutely no memory of the past fights, meaning each battle seems like the first one to him.
so how does sans feel when that expression is nothing but boredom? when his first attack is dodged without even flinching? how does he feel when each of his attack patterns are being dodged with near perfect muscle memory—and as the battle keeps going, he knows he's getting more and more tired and that he can't keep fighting for much longer? no matter what he does, none of his attacks work.
there's no shock, no surprise in their face—just utter indifference, and the impatience of someone who's seen this many, many times before. imagine how he feels as the realization slowly dawns on him that this is probably the time they succeed.