I believe it's a fluctuation of actual value. Your content either has value for a payout or it doesn't. Your power doesn't go down over time because people lose interest in your already posted items. I have noticed from day to day my "dollar" value will be down even after payouts and curation award like you said. I would just suggest you keep at it, trying to effectively grow your power then when it gets big enough try and cash out when it's at a beneficial level. Like the stock market lol