The overall value of your wallet is going down because the overall value (market-cap) of Steem is going down. SP doesn't ensure you maintain a dollar value, it ensures you maintain an equivalent percentage stake in the total value of Steem. If the market-cap of Steem were stationary at say $100,000,000. Over time you would see the price of each Steem go down and the amount of SP go up. Balancing each other out for no change in overall value(market-cap). Right now the Steem price is simply falling faster then SP is going up. I hope that makes sense.