>>21413679doesn't help that the anime hand-waived it as well
"Also, Dedenne's a fairy-type pokemon."
"Oh ok."
Personally I just think that one day, Arceus was bored, and so he decided to sprinkle some of his
kantonian kawaii sauce on some pokemon, made them fairy-types, and then watch the carnage they were causing on a flat screen TV.
Alternatively, you could say that time unleashed something inside some mons that made them into a fairy. Havoc still ensures though