I think BitStamp has changed their USD/BTC exchange rate source.
Yesterday it was averaging 20 USD below the rate shown on coinmarketcap.com - but today it is 10-15 USD above that.
So buying e.g. 1 BTC from them today will be approx. 20-35 USD more expensive.
You got things the wrong way around. Coinmarketcap.com is not a price-authority, it just gathers data from other sites, in this case BitcoinAverage. Just as the name suggests, that site averages the Bitcoin price at several exchanges. Unfortunately, it also includes MtGox in this average, while the Gox price is little more than a random number these days.