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You get more STEEM per dollar. So if you write a $5 post when STEEM is at $5, you only get one STEEM.
If you write a $5 post when STEEM is 50c, you get 10 STEEM, which is ten times better :)
Also when the price drops there are fewer other authors posting, so less competition for eyeballs and upvotes.

Cool calculations thanks!