Well, I guess the simplest answer is that Suethors aren't really that creative. They need to give their characters as many powers as possible, because the character has to cover all bases to not be seen as useless.

My fandom is One Piece, and that series has so many Sues with multiple Devil fruits, you'd think there was a sale on the damn things somewhere. Or worse, they have one Devil fruit that doesn't make any sense. I read one story where the Sue ate a fruit that turned her body into that of an eight-year-old, but in return, she got the ability to fly and all kinds of strange things, making me just scratch my head in confusion.

Oh, definitely. Just the way she forgets about the ghosts, Neferet sneaking around in a black cape and tophat while twirling her mustache and randomly bursting out in maniacal laughter out of pure evil-ness, and the important rituals she needs to hold now just oozes responsibility and the ability to focus on her duties. I guess she suffers from some kind of selective short-term memory-loss.