I was always puzzled by this question as well and never did find a definitive answer. This is my information on the subject:
Vmcg is higher because:
•Does not have the advantage of Vmca’s 5 degree bank towards the 2 operating engines
•Smaller moment arm on the ground (rudder to wheel area) than airborne (rudder to CofG)
I don't have specific data from the 747-400, but on the 747-100 at the example weight Vmcg 118KTS Vmca 102 KTS (HBJp.263, 264)