It is an assumption, like everyone else's, based in part on the fact that antiquarians had encountered the Khnum-Khufu cartouche but not its simpler Khufu form. It leads me to believe that, initially at least, they thought two dots formed part of this representation of the name. They later learned that this was wrong.
I am not behaving like I am "all-knowing". I was, as I stated, simply correcting your error for that is what it is.
You are laying claim to an idea which is not supported by relative spacing and sizing of other glyphs in the cartouche, nor by any evidence of the titulary Khnum-Khufu stacking the water vessel and quadruped glyphs in a horizontal inscription bearing this name anywhere else in the archaeological record.
That is not me nit-picking. That is me using evidence to argue against your claim. I'm sorry that you take it so personally; there is no offence intended.