Your Steward is correct, and also incorrect, unless your building has a policy of choosing open routes by seniority.
As long as they are working someone under you, they have to work you, but you are not guaranteed more hours than them.
You are guaranteed 8 hours if you work, but are not guaranteed to get the best route, or the route getting the most hours.
Extra work does go by seniority, but a route, even those that may get more OT than others, is not extra work.
I also believe in seniority and the benefits that come with it. Ask your Sup why they are not calling you first, and if they aren't, ask them if they could call you first, due to your seniority.
But ask nice, because they may not necessarily have to.