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His Steem power now is 40 and his vote still gives 1cent

If 28 steempower gives 1cent, why is 40 steempower also giving same

You will find the answer in the whitepaper.

Where do I find that, am a noob

You just need to do the most basic google searches. https://steem.io/SteemWhitePaper.pdf or search for steempower and vote. There are countless posts on this topic.