Another possibility is that this individual bought two guns, took the frame and slide that they wanted to make one gun, and put the other frame and slide together to sell that gun. Some people like a certain slide finish, a certain frame color/texture, but they can't find all of those features in one gun. So they buy two, mix and match to get the features they want into one gun, then assemble the remainder into another gun that they sell.
If that's the case I wouldn't mind buying it, but if you ever need to send the gun to Glock for some reason I don't know how they'd react with mismatched serial numbers on the frame, slide and barrel.