Sometimes, checking permissions on a SharePoint site is quite a hassle and I several times had permissions issues especially when working on an anonymous access enabled application (see one reference here) and you don't know where it is coming from.
I found a tool that can be very helpful in discovering the permission details for an item or a page.
See the article on the Rodney Viana's blog, and the tool download on Codeplex
The opinions expressed herein are my own personal opinions and do not represent
my employer's view in any way.