This is almost certainly the case--they use the cheapest Green ones in the Essential cases because there's no speed advantage to a faster drive when the external interface is USB, and the lower power allows for smaller power supplies.
I actually really like WD's Green series (both my home and work servers use them), but the "variable" RPM claim is apparently marketing crap--they're pretty much fixed at somewhere around 5400RPM, not variable at all. The variable might refer to the fact that each drive spins at a somewhat different speed, as opposed to the same one varying its speed.
Regardless, it isn't of much consequence unless you tear the thing apart to use the drive in another case or as an internal.