They're either from curating content (voting) or for writing posts and setting the payout to 100% SP.
The last major steem update changed it so those payouts need to be manually claimed, rather than just automatically crediting you.
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I see I was wondering about the curation pool, since I'm getting rewards all the time.
Yes it does make sense to make it manual to have people more engaged in the website, rather than just to setup bots.