What determines how much of the total available STEEM Pool is actually delivered as rewards per day?

Good question. In short: The reward pool itself.

The reward pool is used to calculate the value (number of rshares) that is added to a post when the post receives an upvote. The lower the reward pool, the lower the number of rshares that is added. So a lower reward pool eventually reduces the rewards being subtracted from the pool itself.
I hope you understand what I mean. It's quite complicated :S