He has the devil's luck. I got a ton of accidental fusions in the original P3 (Mot instead of Sameal? Yes please) but not so much in FES and I don't remember ever getting one in P4.

Be grateful they don't work like the main MegaTen game where they are a bit more common and the rare part is that they fuse into something better than intended, usually you just got a lowly Slime...