I believe you are connecting to a screen, likely with an HDMI, welcome the world of AMD's stupid default overscaling feature. Go into the Catalyst manager and making sure you are in advanced mode, look for the scaling option and set it to "0". It will for whatever reason by default be set to something like 10 or 15%. When you move it to 0, it will fill the screen. Stupidest default setting I have ever heard of and I would like to see the reasoning behind this fairly undocumented feature that is on by default and set to a higher number than most people will need. Dont have an ATI card in front of me or I would screen shot the exact place for you, but trust me, it is in there.
Found a screen shot, looks like this in the older driver version: