Vmc(a) or (g) is a measure (indirect) of rudder authority.
The rudder exerts a force. The effect of that force is greatest when the arm on which it operates is longest.
It is shorter distance from the rudder to the maingear (the pivot point on the ground) than it is from the rudder to the Centre of Gravity (the pivot point in the air). Therefore, on the ground, the rudder has less authority, therefore Vmcg is higher than Vmca.
As it points out, in the air you also have the option (considered complusory, in effect) of banking towards the dead engine. This is not, for obvious reasons, an option on the ground. Such banking also reduces Vmca.