B747-200: Vref, V2 and Vmcg

[forsyh]

I quote from one of my books:

"Vref is included on the takeoff data card as a reference for computing enroute climb speeds. Vref varies directly with gross weight and simplifies the additives for climb speeds while V2 is influenced by Vmcg at low gross weights and would require a wider range of additives for the various speeds."

Can someone please explain how "V2 is influenced by Vmcg at low gross weights and would require a wider range of additives for the various speeds".

Thanks.