I was wondering the same too.

I think, more than the vested funds, it's "its usage" aka activity level that decides the reward pool allocation. I guess, it was after Steemauto, the auto-voting tool on Steem, decided to withdraw support from Steem, the voting activity reduced drastically. And that suddenly increased curation rewards for manual curators.

Although Steem Witnesses quickly put up a copy of SteemAuto, it has limited features. And since most of the people had lost interest in Steem and were already powering down, they had no interest in participating by signing in to a new tool. Some users were inactive for months but their voting was active but Steeauto's dicontinuation stopped that too. This along with conversion of Vest into liquid STEEM probably contributed to a good RoI for Steem curators. Dunno, how long that will last!