I'm not quite sure why the discrepancy...back when I started casting and reloading the Lyman #3 Manual said to size 357 magnum bullets and 38 special bullets to .357 diameter....so that's the dia. sizer I bought and used for 33 years. I purchase the new #4 edition and it says to size 38 special bullets to .358 and 357 magnum bullets to .357...didn't say why. I just purchased a .358 dia. sizer and will conduct some trials with 38 special loads sized some .357 and others .358 and see what happens.
I'm thinking it's going to be six of one and half a dozen of the other.... but you just don't know until you try something....my loads just might go into a ragged hole when sized .358.
I'm thinking the only difference is going to be .oo1 inch.
Gary