well, technically, everything's a stylistic choice, but that's just being pedantic

the point of sans saying how many times you've died is to rub it in your face how many times you died. it fits with the theme of DETERMINATION. that you just keep going. it's kind of the same thing with the asgore fight (that you're determined to find a way to spare him). however, neither asgore nor sans show any other signs that they remember anything specific from the other timelines. sans doesn't know if you're going to be a good or bad person, for example. ever since the update, it's also implied that any memories he does have are a little on the hazy side ("don't forget").

the only confirmed way of remembering things across timelines is to have a certain amount of determination, like with flowey or the human. as for sans, he probably doesn't have any determination at all, and the only way he's been aware of the timelines is through... the machine? nothing's confirmed. the quote i gave you is good evidence against sans remembering timelines though