"Match desktop size to:" Not Working

As it says above, this option doesn't work for me. When ever I airplay and choose "Match desktop size to Apple TV" it stays matched to 'This Mac' (the option shows Apple TV is checked). It's not the end of the world however, when matched to Apple TV all icons, windows, etc are a larger size and easier to see when airplaying to my 47" TV.

Any help on this would be appreciated