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The first payout happens sometime between 12 and 24 hours from when the post is published, depending on when the votes came in. For example, if a post has hardly any votes 11 hours after it was posted, it might say "pending payout 1 hour from now" but then if someone with a lot of Steem Power votes on it, that will extend the time until payout. After that vote, the time until payout might increase from 1 hour to 8 hours. If any votes come in after that first payout, the process starts over and the next payout will be several weeks later.

Ok, gotcha.

What is the reasoning behind that?

I'm just postulating here. Maybe the first payout happens relatively quickly so content creators can get paid more quickly. Then the 2nd payout, weeks later, ensures the author still gets paid if anyone gets value out of it post-payout.

There is a first payout in 12 hours plus (depending on how many votes the post is continuing to get. Then, after the first payout, there is a second and final payout after 4 weeks for those who discovered and voted for it late.

So the first payout is incentive to contribute, and the second, weeks later, is to cut down on fraud?