We need to clarify which are the differences between an organization/site owner and an organization/site admin managing the user membership.
Basically these are the differences:
1) A Site/Organization Administrator cannot delete the membership of any other Administrator or Owner.
2) A Site/Organization Administrator cannot remove the Administrator or Owner role of other administrators/owners.
3) A Site/Organization Owner can delete the membership or remove the roles from any other administrator or owner.
In case of an organization has an associated site, the Organization owner can also delete the membership or remove the roles from any other administrator or owner from that site.
More information in the issue
LPS-31791 which fixed and established this membership policy.