I'm going to seriously oversimplify here, but basically the longer the proof frequency you select the more KOIN you are likely to "win" when you submit a proof. You can think of it like the extra time lets you build up hashes to "bundle" into your proof. The hashes are also more valuable because the miner automatically increases the difficulty. So the longer the proof frequency the more, and the more valuable hashes, you get to include in a proof, which gives you the chance at winning more KOIN than if you had a lower proof frequency. The downside is that it's still a lottery, so you only MIGHT win more KOIN. You might also win less. So reducing the proof frequency reduces the variance. It's a law of large numbers thing, the more proofs you submit the more your average reward will reflect what you "should" win based on the probabilities.