>But that may just be because it's running more frequently to compensate for the yellow line.

This is the case. There is no redundancy on green, and those people are also impacted by significant delays if their journey was originally to be on Yellow across the river, so green has been somewhat prioritized.

It's also why they're running Blue and Blue + trains, with the latter being the Blue line except where it diverts and covers the Yellow line tail in VA to Huntington.