Maybe it's simply that they had to leave those that were saturated that way. Imagine if you'd played 900 rounds when the change came there would be two options; a big hike in your average or another 800 rounds to saturate.
I don't really know but it's possible that when a score replaces the highest included in a saturated average then the latter is deleted (only 200 in this case were ever kept) meaning the only other option was to make all champions play another 800 regardless of how many they'd already played.
I think when Champion was introduced they did something daft like discard the lowest 200 of 400 folk had in saturation, making averages go up, and require another 200 rr's.