Ah, for some reason I didn't notice it increased over time, though it only went up per wave and with some effects. Makes sense.
One more thing, what exactly counts as "ace a whole zone" for the ship unlock? I thought I had a run where I had "flawless" on every wave in first zone (I assume you get that for not taking hp damage) but it didn't unlock. Of course I might have missed something, but it's also possible I'm misunderstanding the condition.