spuser.IsSiteAdmin get different result because of user policy

This make me confused!!

I create a user "test" in AD, then in CA, I modified specified web application permission policy. Make sure "Full Control" permission level checked "Site Collection Administrator" option. Then I modified same web application"user policy", add user "test" with permission level "Full Control".

When I logged in SharePoint with user "test", SPContext.Current.Web.CurrentUser.IsSiteAdmin return TRUE.(user test)

But when I check the property by SPContext.Current.Web.SiteUsers.GetById(id).IsSiteAdmin, the same user will return FALSE.

Anybody can explain this ??

Yungfu