That's so that your Steward 1 Pilot 1 can function as a steward while also the pilot, because he functions one level lower in both.
Unless he's Pilot 2, he cannot work as pilot if he works as steward too (as the pilot needs a minimum of Pilot 1), but your point keeps...
I don't recall mid passengers requiring a steward.
Am I in error?
Not in CT.
In MT (RM, page 82), the crew formula for stewards is 1 per 8 command creo or high passengers and 1 per 50 non-command crew or middle passengers...
In MgT, one skill level, 0 skill level included (not one steward) per 5 middle passengers