In the 3 part novel series (FNaF: The Silver Eyes, The Twisted Ones, The Fourth Closet) we are introduced to discs, created by William Afton, which can make you see things, how you think you see them.
William uses these discs to make the Twisted Animatronics appear to be made of flesh.
And it is speculated that these same discs are used to make the Nightmare, Toy, and Withered animatronics, look the way they look.
In FNaF 3, we can see a box of dismantled toy animatronic masks, and withered foxy's arm.
Scott even dedicated a teaser to this box, meaning that it could have some importance to the lore.
What if, before springtrap escaped the fire, he grabbed one of these discs that were in the box, so that he would look human to everyone else, but he would look like a slightly different, nightmarish (notice scraptrap's pointy teeth) animatronic to Michael.
It would make sense, after all, springtrap's suit isn't the only thing that has changed, William's corps has totally changes as well, he doesn't even look human in FNaF 6.
(The discs being in the FNaF 3 box could also explain the phantoms, also, mangle replaces freddy, could this mean something?
Who knows, this is just speculation after all.)
Thanks for reading.