Three Five Seven, the answer is probably too simple...
The .357 SIG is a European designed round. Which means the designers think in millimeters, not caliber (hundredths).
For contrast, the .357-.44 Bain & Davis was a .44 Magnum case necked down to - no peeking - .357" diameter bullets.
SIG was already making .355"-.356" bore diameter barrels. Why bother with different barrel making equipment to cut a bore only .001" to .003" larger?