Dell U2717D and mid 2010 MacBook pro

I have connected a new Dell U2717D to my mid-2010 MacBook Pro. It works fine, but the screen resolution is nowhere near what the monitor is capable of (2560x1440). I have tried to change the display settings in System preferences, but it really does not make much of a difference. Is that due to the screen resolution of the macbook, which is (1440x900)? Or have I missed a setting somewhere? I'd be grateful for any wisdom on this.