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They take their vote out or unvote you.

By the looks of it you did not get downvoted. So it could have been 3 things. People removing their votes, the voting power of people who voted on you dropping, or the steem price dropping.

Thanks for your response.. There is no chance of first point . Because vote count is still there.. But it may be due to second or third points ..

Today it happened to all my posts same. That means these earnings are not sure until you withdraw them?