I recently ran into the weird issue of not having my tabs highlighted correctly. After consulting Uncle Google, it seems Andy Burns ran into this same issue.
The problem started after creating multiple aspx pages within my site and then setting up a tab for each page by editing the Top Link Bar (also known as the top navigation) under Site Settings. Each of the tabs worked correctly, but after clicking a tab that tab would not be highlighted and the home tab would stay selected.
The fix, as Andy mentions, is to change your URLs to be relative. After the change, your tabs work as you would expect.