It includes the Pelgrom coeff for the mismatch model of equally sized devices.
In general, this is your real limiting factor when designing high speed applications which require two or more identically sized devices. In reality because of process non-idealities these devices will be slightly different in a percentage inversely proportional to the device area. Device mismatch will determine some key performances as for example the input "offset" or even harmonic distortion (also for fully differential circuits). Therefore, in order to minimize the impact of mismatch on your circuit, you need large devices which goes at the expense of more power consumption for the same desired speed.
nathan