Network file permissions dont accumulate

Am I going crazy. I thought file and directory permissions were additive. If you were in two groups and they had different permissions for the same directory (neither are no access) then the resulting permissions are the sum of the two. Is this affected by trusts. I have a resource server in a resource domain that is in a trust with the login domain. If a person is in two groups their permissions for a directory are the least of the two groups permissions. Set me straight, please.