Since no one else has responded yet, I'll jump in and put out my speculation based on my experience with 9mm P228 and P226 mags since I don't own a .40 P226. I suspect that the differences in mag capacity is based on whether they are Sig-branded mags or ones with the Mec-Gar name on them. I have Sig-branded factory P228 mags made by Mec-Gar with a 13rd capacity and some of the equivalent Mec-Gar branded ones that have a different follower and spring that increases the capacity to 15rds.
I think that is what is probably what is going on with the .40 P226 mags. The standard Sig factory .40 P226 mag is a 12 rounder. I suspect the 13rd mag in your first link is the M-G branded version of that Sig-branded mag with a slight modification - lower profile follower?, shorter mag spring?, thicker mag base? - I don't know. The 15 round mag in your second link appears to be the the 13rd M-G mag with a +2 adapter base.
Last edited by wgsigs; March 20, 2013 at 09:25 AM.
Reason: removed reference to 14rd mag not mentioned by OP