The following post from Jon Galloway goes into great detail explaining what I think it is you are asking.
The article includes a class diagram that shows the relationship between all three classes that you have listed.
The relevant take-away quote is this one:
SimpleMembershipProvider is an implementaiton of an ExtendedMembershipProvider, which inherits from MembershipProvider and adds some other account / OAuth related things.
Be sure to look at the class diagram, it is in the section called How SimpleMembership integrates with ASP.NET Membership
Hope that helps.