Special Operations Within Special Operations

The all-seeing vid-eye of GameTrailers has nabbed some more Modern Warfare 2 footage, with explanatory chatter from the devs. It focuses on Spec-Ops mode, which is the co-op mode which consists in a whole bunch of independent missions that boast a modicum of internal variation. Not sure I quite understand what this chap is saying about alternative difficulty setting, however. He says the difficult effects the AI most, and yet you can both select different difficulty levels in the same co-op game? Hrm.