Hi,
I'm beginning to think you have something else doing that redirect like an IIS module, or javascript in the content on the page that does the redirect. I would use a tool like fiddler, and see what exactly is going on, is it really redirecting, is it a 302 or a 301 redirect etc.
Url manager doesn't redirect, those mappings are for url rewriting which doesn't do any redirect.
I would look into whether you have any IIS modules like the IIS Url Rewriter which can be configured for redirects.
Hope that helps,
Joe