Some of the Powered down Steem is being recycled right back into Steem Power by way of upvote bots.

I'm an example of that. I've been powering down for 3 weeks now and yet at the end of each of those three weeks my Steem Power is higher than the week before. The reason is I spend all available SBD on post promotion and take the power down to keep promoting my posts at the start of the next week.