More simply, in a second-price auction, there is 1 winner, and you clear at the 2nd highest bid.

With 8 winners, they should be clearing at the 9th highest bid.

Of course you don't have to do it this way, but it's the least game-able outcome.

It's clear they had the second-price/Vickrey style in mind, and they came awfully close to getting it right.