Perhaps their (RF) grounding wasn't quite good enough, and they needed a CMC on the line cord as a whole? Perhaps it was an errant guess and including ground wasn't necessary, so they cost-optimized it in the next revision?
Ground filtering isn't usually necessary because ground can be used as a reference plane. Grounded metal enclosure, or metallized plastic, for example.
A case where it might be necessary, is where that reference plane sucks. PCBs with plastic enclosures for example, may have unsubstantial grounding, lengthy tracks near noise sources, etc., thus putting CM noise into all the line wires, ground included. The only solution then is to pray you can put enough impedance in series with the whole cord to get the margin required.
The equivalent circuit, for all wires emitting common mode, is where the bulk of the circuit (chassis, PCB grounds/planes, other connected devices -- keyboard, peripherals, ports) has one [average] potential, and the line cord has a different potential, thus emitting common mode through all wires. A two-line CMC will not help in this case. The usual solution is to CMC the other ports, or to use a shielded enclosure so that all the ports can use the shield as a reference plane.
Tim