Something I think a lot of people forget about Sans is that he’s not like Flowey, when he reacts to changes in the timeline, it’s not cause he remembers, it’s because he notices the evidence in your reactions. He’s just that perceptive. Also, in regards to his weird abilities, they’re all space based (as in teleporting and stuff, not outer space), which I think is a nice contrast to Gaster having time abilities.

adamency:

Yeah, all that angsty stuff where he remembers every timeline is…well, inaccurate. Non-True Resets seem to leave echoes among everyone who’s met Frisk, but Sans’ knowledge of timelines means he can take these echoes at more than face value. And he’s incredibly perceptive and can even usually get the count of how many times you’ve died based off of your expression, although at the higher counts he starts getting it wrong and gives up.

It’s interesting to note that another character, who actually has no knowledge of resets or anomalies to work off of, can do this to an extent.

She sees Frisk’s facial expression after they’ve killed her and reset, and zaps them on the spot.

It’s worth noting that people who are able to accurately read others’ facial expressions (particularly with the pinpoint accuracy these two do) often are highly empathetic individuals.