Basically there is a kind of 'super-linear' compounding effect when it comes to steem power and calculating rewards. For example, when I upvote you it gives you about $0.15, because this post currently only has $0.01. If I upvote a post that has $1000, I might add $10, or something. Basically the more rewards you have, the more new votes will increase your rewards. Not sure exactly what the reason was for that design choice, but it is what it is.