It's either your card or your monitor under the "scaling". You'll find it in your video card settings under "monitor scaling" or "scaling"....
Basically, the resolution you have set in-game doesn't match the monitor. How it displays will depend on the monitor scaling selection. Either it will "stretch" the pixels to fit the monitor (this blurs pixels and is not ideal) or it keeps the aspect ratio and resolution, but centers it on your monitor.
You can set your AH to play at your monitor size. If you don't see it in the AH video settings, check "show all resolutions"
And if that doesn't work, close AH and go into the AH2\settings\video8.cfg file and edit it in notepad with the proper resolution. Save, and restart AH and it should work fine.