Isilon Shares, Domain Joined, Builtin users

I need some help understanding where the BuiltIn/Users come from after a share is created. 

For example.

I create a share on Isilon, and from within the Isilon GUI I for the SHARE, set Everyone FC, and add our SecurityAdmin group with FC/ROOT access.

After the share gets created and I review the properties, I'm seeing Builtin Groups... Users and Administrators.

There are other teams indicating they are able to browse and access files/folders (when it was believed they shouldn't have) due to the BuiltIn Users group.