https://steemit.com/steem-help/@jerrybanfield/steemit-updates-summarized-from-launch-to-19-0

It's part of the quote I used above, but goes into more detail (which is what you are looking for)

It's not a fixed rate. It's a percentage that can change. In general holding steem power always means your power increases the majority of the time. Read that blog post for more.