Well you're both kind of right. The thing is, most people aren't going to use the Mobile style unless they are actually browsing the forum on their phone. You could use the default style while on your phone, but then your posts won't show that you were on your phone. So the idea is, if the default skin (style) is easier to use when browsing on your computer, and the mobile skin is easier to use when browsing from your phone, people will use the correct one, and the "posted via mobile" will be pretty accurate.
The last style you were using is stored using cookies. That's why when you come to the site on your computer you still see the default style, and when you come to it on your phone, you'll see the mobile style if that's the last style you selected from your phone.