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It's annual interest. The 30 days is about the distribution between authors, witnesses, etc over the past 30 days. And yes, the ratio between VESTs and STEEM is changing so that you Steem Power holdings will increase even if you do nothing. STEEM is being inflated all the time with new currency creation so holding Steem Power is a way to protect against that inflation somewhat (so it's more inflation protection than interest).

Oh thank you for clarification! I think i got it. Number of Steem Power remains the same in my wallet without any rewards but the ratio to inflated STEEM is changing constantly or the value of STEEM is declining through inflation. There is no such thing like mining or POS which would increase my number of SP.