I'd have to guess the steel or the heat treatment done to the steel. Just because they look the same doesn't mean they are the same.
The cases are the same other than the length. You can load a .38 Special to the same OAL as a .357 and you'll get the exact same pressure in each round.
There are some 38 Special Ruger GP100's where it is generally assumed the cylinders are exactly the same other than the chambering. I would not make this assumption with any S&W.
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