BMM389EC,
This is common to all aircraft, not just Airbus. If other manufacturers don't do it, then they're just simplifying things in keeping one common Vmd speed.
With one engine (or more) inoperative, the entire drag curve changes, and Vmd is at a lower speed. This is due to the addition of two new forms of form / profile drag that are not present in normal all-engines flight, namely -
(1) Windmilling drag from the failed engine, and
(2) Offset Rudder drag due to constant rudder displacement during assymetric flight. (If the manufacturer chooses to use up to 5° of slip to aid control and reduce drag, this is bundled in with rudder drag).
These 2 items of profile drag increase pretty much with the square of the speed, whereas the Induced Drag decrease around about Vmd is much, much less.
Actually, the 10 knot differential is "about right" for most jet aircraft, give or take a knot or two.
Regards,
Old Smokey