Girl at 13, boy at 12?

The gemara in nidda 45b quotes R' Shimon Ben Elazar who has the unique opinion that a girl becomes bas mitzva at 13 and a boy becomes bar mitzva at 12. The gemara explains their machlokes. When I first read the gemara I assumed that the machlokes was just with regard to nedarim and the question is as the gemara explains who "wises up" first. Is it the girl because of her bina yeseira or the boy because of his street smarts. However, when it comes to who physically matures first and when they are chayavim in mitzvos and able to be punished everyone agrees that it's 13 for a boy and 12 for a girl. I was shocked when R' Tendler said that it's a general machlokes but again he was right as Tosafos (d'H eilu) makes clear. The gemara on 46a says that the machlokes also applies to miun so obviously they are not having a narrow machlokes by nedarim but a general machlokes by kol haTorah kula.

I wasn't satisfied though because it just didn't seem pashut pshat in our Gemara. I was happy to see that the Artscroll referenced a Chsam Sofer on this topic. I looked it up and he quoted R' Chananel who says that their machlokes is only by nedarim and not by anything else. That made me feel better but I really had to daven so I didn't have time to read the entire thing. If you want to see how he explains the gemara on 46a, ayim sham ;-).