external display won't use native resolution

I have a 2011 MBP connected to a Dell U2711 that does 2560 X 1440 via DisplayPort. With display spanning, the Dell properly displays at 2560 X 1440. If I select mirroring, it reverts to some lower resolution, presumable the resolution of the MBP. Is this normal behavior? I thought the whole point of an external display was to replace the one on the laptop. I did in fact intend to use this monitor as the main display for the MBP and then use a touchscreen display I have for my PC. With this lower resolution though, I might as well just use the MBP's display. Is there any way to get the display at the correct resolution in mirror more? All the resolutions are listed so pressing alt (option) doesn't change anything.