My logic was that it changes the order in which the AUs are applied to the original Undertale. It was clearly apparent that Alterswap and Alterfell were their respective AUs layered onto Altertale, so I decided to see what would happen if I did the other way around.



Swapalter, for instance, puts Queen Toriel into Blueberry's position, retaining her purple color scheme and having her eyes turn orange for the same reason Swap!Sans's eyes are blue (glow eyes and whatnot), and as with Altertale, has her personality as 80% Blueberry and 20% queen Toriel.



Same goes for Fellalter, but with Underfell instead of Underswap. I based her design on Alterfell!Toriel (basic look), Underfell!Toriel (main color scheme, red stripes on the dress), and Underfell!Sans (gold tooth, permanent eye glow). As for why blue instead of red, blue fire is hotter than orange fire.



But yeah, it is funny that the order of the words in the name is that important to what AU it is.