What if Soda doesn't really always wear flannels?  It doesn't specify that he does in the book, he wears them like twice in the movie though. But what if it was just laundry day at the Curtis house and he's like, "I don't have any clean clothes except these 3 flannels.."  This is a really crappy head canon and I've had writers block, but it's something that just popped into my head so why not?👻