Generally speaking, as you move "up the ladder" from PT to TCD to pkg to feeders, your new driving position involves a progression. If you are making more than the start of the progression, you don't take a pay cut. You stay where you're at until the progression catches up to your current rate. Then you continue until top pay.
You are too focused on the "part time" in Part Time Cover Driver. And you feel that makes them equal to you as a part timer inside the bldg.
Is this the scenario?
You, employee A, have more part time seniority than part timer B.
B signs the bid list and goes TCD. A either doesn't sign, doesn't qualify, or whatever. A stays in hub.
B has bid into a job that pays $28. A stays at his job making less, for example, let's say $15.
A & B both sign Feeder bid list and go to Feeders.
A enters feeder progression and goes to $18.75 and finishes the progression from there.
B stays at his current $28 until the progression catches up/passes him. He/she doesn't have to take a pay cut.
That's how it works. Different paths that end up at top pay if you both stay long enough.
As you seem bothered that you both currently perform the same work and there's a pay difference, consider this. Employee B was in pkg doing the same work as a full time package driver for about $5/hr less and 20 hour vacation checks when they worked over 40 every week. Also accumulating no full time pension.
The fact that you looked into it and the Company or Local had mixed-up or confusing info does not mean anyone changed the rules. It means that someone or several someones isn't familiar with the contract, which won't surprise you once you've been around long enough.
I hope I understood everything correctly and the explanation helps you understand.
And Welcome to Feeders! Be safe out there.