Yes, as Dave has said it's the isolation angle rather than the voltages.
The single-gang size 240V only device is connected directly to the supply, with a small fuse to enable it to be used on existing lighting or power circuits:
The dual-voltage types, on the other hand, have the required isolating transformer for use in a bathroom. Notice that the plate is the usual two-gang size, to allow room for the transformer:
It just so happens that all of the isolated types sold by TLC are dual voltage. It's quite acceptable to install a 240V-only outlet in a bathroom, so long as it has the isolation xfmr.
THe dual-voltage types are quite handy to cater for tourists from the Americas in hotels and similar places.
Notice that the receptacles will accept different plugs. The 240V side takes British and Continental round pin, plus Australia flat-pin. THe 120 side accepts Continental two-pin and American NEMA.