That is the "guessed" amount. I think this takes the value of Steem Power into account, should you power down. And it also sort of guesses how much is still open to be paid out.

Since I want to grow, I'm not powering down (yet), there will some extra income over the coming days from earlier posts. Also I delegated some Steem Power to #minnowbooster, which pays a bit each day without having to do anything for it.

But what I can trade right now is 3,633. Like I said when people upvote my posts this amount will increase, this will increase the payout.