I think you are conflating the original 9mm 229 with the 229–1. SIG was making the original 13 round 229 for over a decade before the switch to the 229-1. They’re not the same gun.
The 229-1 mags do indeed hold 15 rounds, because they were made to fit the increased internal volume of the 229-1 frame, which is really the .40 cal frame.
However, the original 9mm 229 which replaced the 228 used a 9mm specific frame which used the 13 round P228 magazines. Hence the “-1”
229-1 mags won’t fit in the original 229 9mm frame (or the P228 frame).
Yes 13 rounder rattle around in the 229-1/40 frame but IME they work, at least well enough for range use. Same with shooing 9mm out of the cheaper and plentiful 40/357 mags.
The real motivation behind the -1 was streamlining production to a single frame for all calibers to save money at a time SIG was trying desperately to find ways to cut production costs on the classic P series guns.