The issue with big differences in 24 hour average rate and the spot rate are not that common, I know recently SR had it's rate at a much higher than the market rate for a few short periods recently when the price of BTC dramatically dropped. For the market to be stable (ish) hedging is a necessity to even out some of the volatility, to protect the vendor and the buyer from big variations. This is why SR charges hedging fees on all sales so that money essentially funds an insurance against what you have just described. When the price of BTC is going up those hedged funds are making SR a lot of money as only the dollar value is released to the vendor, not the amount of coins. However when things swing in the other direction as they did recently that pot of money would have been used to offset the potential loses.