Because Frisk Does Not Remember A True Reset. On Normal Resets, Frisk Turns Around And Shakes Sans's

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.

Undertale fandom in one sentence.

Story of my life

I love how it’s in an infinite loop because of resets!

Happy 8th Anniversary, Undertale!!

Thanks for eight years of new friends and fun stories!

My undertale soul colour headcanons

-Monsters have soul traits as well, but they are too desaturated to be visible. Determination is not a soul trait but a substance that saturates the colour of the soul. Humans, who have a lot of DT, have very bright soul colours.

-A human's eye colour matches their soul's. Since Chara/Frisk (Depending on who you think was to blame for the red eye colours in the soulless pacifist run), have red eyes, and eyes are the window to the soul after all.

-Hot take: If Sans had a soul trait, it would be Justice, not Patience

Most fanworks associate Sans with patience, but if you think about it not much about him relates to the trait. We give him that because he's blue, lazy, and doesn't do anything. But that's not really patience, just laziness, and probably depression.

Sans is much more intertwined with the concept of Justice. He judges the player for every EXP they've earned. His very magic delivers karmic retribution. He fights you only on the route where you kill everyone. The yellow eye is usually skimped in favour of the cooler (ha) looking blue eye, but I personally think it has a lot more lore importance.

(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.