Id speculate the S&W did that to cut down on mistakes and errors in the ordering process.

Let us hop in Doc Brown's Delorean for a trip to the 1990s. Assume in this alternate timeline S&W named the 6906 and the 6926 "properly" . We'd walk into the 1990s gun store to find an irate guy yelling at the hapless owner. Turns out three days before the owner phoned in an order to S&W for a 5923. Not a problem, except the customer paid for a 6923, which is a very different firearm. Oops.

Even in this universe, things get wonky with S&W model numbers. I can't count how many times ive called dealers looking for S&W 3rd gens and they've said something like "umm yeah we'll keep an eye out for a 5903. Oh, uh, what's a 5903? Thats the .45 , right?"