Hello Delford,
I was able to reproduce this behavior and have checked it with our development team.
Regrettably, this is not a bug but behavior caused by the design of application.
In this scenario, the following 2 namespaces come into the game: CMS.Membership vs OM.Membership
The macro engine removes the prefixes and therefore it returns a completely different collection that you are expecting as it's related to On-line marketing.
In this case the following macro will work:
{% CurrentUser.OtherBindings.MembershipUsers.Count |(identity)GlobalAdministrator%}
Best regards, Martin