Question on IP encapuslation

I have a question on the IGMPv2 draft. Based on the spec, there should only
be one copy of IGMP running on a physical interface even if there are 
multiple IP networks running on that interface. correct??

My question is what if the IP networks running on a single interface have
different encapsulation type. Shouldn't we run IGMP for each encapsulation
type even if the underlying physical interface is the same?? If so, doesn't
this conflict with the spec??