Booked hotel in Germany - will rates stay the same?

Living in the US, booked a hotel through Expedia.com for Germany. Expedia has charged me already - looked up prices for the same hotel and dates through the actual hotel website and its the same numerical price but in euros. They stipulate a difference because of exchange rate, but what's the deal with the same price only in dollars (expedia -$90 US a night) and euros (hotel website - $90 euro a night)?!