I'm pretty sure that's how it works. I know someone that I can ask for sure who runs a couple hundred TB across a variety of drive sizes. But he won't be available until later in the evening.
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EDIT* I just spoke to my guy and he's stating the oddity of documentation is correct as you mentioned. So in your example, the 10TB drives would completely fill before the 20TB gets used again. So, not great. His recommendation is to go with 'most free' and use your split levels to manage groupings of media.
My bad for misunderstanding. I have all identical drives, so it's not an issue for me, but now I think I'll go switch mine over as well.