Quotas are applied to the Everyone group and the BUILTIN\Administrators group differently than to other Windows groups.
If the quota target is BUILTIN\Administrators, the entry is considered a user quota, for tracking only. You cannot impose restrictions on BUILTIN\Administrators.
If a member of BUILTIN\Administrators creates a file, the file is owned by BUILTIN\Administrators and is counted under the SID for BUILTIN\Administrators, not the user's personal SID.