I just found this issue on the latest 4.6.21 release but was able to reproduce on 4.6.18 as well.
When I create a new smart group:
Search -> Advanced Search -> Choose any group but Administrators -> Create smart group
then in the next step I want to edit it:
Contacts -> Manage Groups -> Choose the one you just created -> Settings -> Edit smart group criteria -> On the top of the page Edit xyz smart group criteria
and I see Administrators Group there not the one I chose originally.
This seems to be a bug to me? Can anyone confirm or test on previous releases?