Web sites use a variety of mechanisms to decide to serve a mobile version of a page; the user agent is just one of them. They may set a cookie or some other piece of state that will always redirect to the mobile page after you've visited once. They might look at screen dimensions and do it that way. It's very hard to say what this particular website is doing. Maybe try clearing cookies?
Are you certain that the UA you have set is a desktop UA that the site recognises as a desktop browser?