When you got resolutions higher than 5760x1080, it means your Eyefinity setup has some "bezel adjustment" in place. I do have it but I adjust it manually.
But what you can do is to check how the images fit around the bezel. If some part of the scene gets hidden behind the bezels, then it means you really got bezel adjustment. It is a good feature to provide better image. If the bezel adjustment does not look good, you can still manually adjust it in catalyst.