They unfortunately didn’t fight to end the two tier system for part time workers, which is why your math is off. The part time employees aren’t very engaged with the union and there was/is little attempt from the union the reach out to that section of their members to educate and involve them.

The union leadership has a vested interest in selling that they got an amazing deal, but this was a huge failure to fight over. Two teir pay is used by the owners to ensure their employees aren’t a united front.