Hm, this seems to be a bug - It is supposed to show as “owner” when the currently logged in user is the owner of the group, and “member” when the current user is a member… but it seems to show “owner” for everyone regardless of membership. Will fix.
EDIT: I see what’s happening, and it isn’t as bad as I feared, actually. The display logic is checking if the current user is a group owner, which includes all admins. The display here should still display the correct membership on the card, admin or no - thanks for catching this. 
https://github.com/discourse/discourse/commit/3d1ff8e0ad63aac76519c0e3b47d5dee64653a77