Secondary Groups for users

I have an interesting situation, I have two users user1 and user2.

user1 is primary group is oinstall, secondary dba

user2 is in the primary group staff, secondary oinstall

I have a script with the following permissions:

 -rwxrwx---  19 user1   oinstall   18432 Feb  4 12:30 script.sh

user1 can execute it, user2 cannot.

If i change the script group owner to staff, user2 can execute, also of i were to change user2's primary group to oinstall he can execute it.

I would think that if user2 is in the group oinstall and the group has execute permissions, user2 should be able to execute, am I missing something?