When they say 2 1/4 or 2 1/2 inlet or outlet, they are referring to inside diameter. If you were to swedge the tailpipe over the outlet, it would be a very odd size, and you'd have trouble finding a clamp to fit. Your guy probably left the front pipe a little long, then had to shove the tailpipe in too far to clear the axle. It's going to take some heat and muscle to get it apart and straighten it out. Good luck
Right you are. Exhaust pipes are usually outside diameter and a 2 1/2" id muffler should fit over a 2 1/2"od pipe. Also the metal thickness for mufflers is thinner than the pipes so it makes sense for the pipe to be inside them. The clamp crushes the thin pipe to the thick one. toolman