Channel length modulation
Channel length modulation causes a finite ratio d(Vds)/d(Id) which resembles a finite otput resistance ro. With other words: The drain current Id is not completely independent on Vds (Id rises slightly with Vds). This output resistance ro acts in parallel to the gain determining external ohmic resistor Rd. As a consequence, the ouput signal voltage is determined by the value ro||Rd (which is smaller than Rd).