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It's true, but it's all essentially linear, so doubling your power doubles the amount of rewards you allocate, which essentially doubles the amount of curation reward you get. This breaks down a bit for small votes, like those under $0.50, because once they're too small you'll lose some to rounding errors.

Thanks for your answer @biophil. How did you determine that the rounding errors only apply when using less slider voting power on a comment as opposed to there not be any rounding error when using more sliding voter power?

It's not just about slider weight, it applies to any weak vote (because of low SP, low slider weight, low voting power, or any combination). The reason is that Steem amounts come in exact multiples of 0.001 STEEM. If you're due a curation reward of 0.0099, you'll only get 0.009, which is a 10% loss. On the other hand, if you're due a curation reward of 1.0099, losing that last 0.0009 doesn't matter as much.