I’d be betting, like Norm, that the dimensions on the muffler have been altered by the welding process.
They are made of thin gauge steel so distortion is just about a given with anything approaching a red heat.
The spring clips anchor over the seams of the muffler and pull it toward the exhaust spigot. I am guessing that the welding process has resulted in a reduced distance between the seam and the spigot.
