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Because Frisk does not remember a true reset. On normal resets, Frisk turns around and shakes Sans's hand before they are told to. The only reason this interaction would not occur on a true reset is if Frisk didn't remember it.

If you go back to the game after a true pacifist run, Flowey says "Let everyone be happy. Let Frisk be happy." A true reset goes against what Frisk wants. Normal resets are performed by Frisk. True resets are performed by YOU.

(Note that the question is HOW aware, not IS he aware)

EDIT: I think my question may have been somewhat misinterpreted based on the responses.

The "all his knowledge is just based off observation" choice implies that he has LESS deja-vu than other Undertale characters. If you think that Sans has no more and no less deja-vu than monsters like Toriel (Remembering your pie choice) and Undyne (Getting a feeling you might have been friends before), pick the second option. Choose the first option ONLY if you think he has a weaker/non-existent sense of deja-vu compared to them.