You are confused. Steem Power is measured in Vest (see steemd.com). The number of vest (steem power) does not change unless you curation or blog. What you are seeing is the conversion rate of Vest to Steem that changes constently. As time proceeds, it takes more Steem to buy a Vest. When you power down, the Vest is converted back to Steem.
Steemit is showing the Steem conversion rate. Go to Steemd.com@stealthtrader and you see the number of vest unchanged each day.