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would that affect how much reward author is getting? or is it only affects the voter?
For example, I see threads with about the same number of upvote, but significantly different amount of reward for the thread. Is that because people with different steem power voted for the threads or is there a different reason behind it?

The reward is determined by the amount of Steem Power the upvoter has. Say I had 200,000 Steem power and I upvoted your post. You would get a larger reward then If I upvoted it now with my only 1300 steem power.

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