That may be true for STEEM and STEEM DOLLARS, but I'm pretty sure STEEM POWER accrues interest in addition to being impacted by market forces. If you go to https://steemstats.com, put in a user name, and go to the Steem Power tab, it should show you the accrual rates and how much interest that individual is accruing over various time periods.

I'm pretty new here though, so I could be mistaken.