I had the same issue you're having. It doesn't affect performance, but it's just enough of strange thing that it makes you think something is kind of wrong.
On the AMD forums/website, they will tell you to plug your monitor into the other DVI port on your graphics card to get rid of this issue (you're using a DVI connection I'm assuming, right?). When I first read their suggested solution, I thought "how stupid", but believe it or not, it works!
The issue primarily affects ATI multi-GPU setups and later driver versions. Since switching the port works, you can pretty much conclude it's not some quirk with your monitor.
Switch the DVI port you plug the monitor into and let us know how it works out.
I really think something driver-related is a little off.