Board Thread:General Discussion/@comment-27524978-20160210030742/@comment-27701762-20160401022831

He does break the fourth wall at other points: when you're hiding behind the lamp, he turns and faces the screen after making his puns, which wouldn't make any sense unless it was as a small break in the fourth wall.

But even then, being explicit about it wouldn't be a break in the fourth wall, as saving and reloading is part of the game's mechanics. Flowey explicitly tells you that he remembers the things you've done, even across save files.

And the comedic lines come after the count. Those lines could come without being led up to by "that expression on your face." Since only the first two lines have to do with the character's expression, they are essentially independent of how Sans knows. In other words, with only a couple exceptions, the exact same lines could be delivered without Sans needing to state that he is counting your deaths by looking at the expression on the character's face.

Which then brings us back to the original question: if both he and the character remember the number of past defeats, then the whole "I can tell by looking at your expression" is a lie and a pointless one at that, so why do it at all?