You might be looking at different room types, or different rates. Or, I frequently find that Priceline and Expedia show a certain rate, but once you click it, they will actually ping the real rate and say something like "rates have changed recently" or something like that.

If you can legitimately find an advertised price lower than the hotel's actual website (roomkey.com is owned by the hotel chains themselves), then you can even get free and deeply discounted nights:

http://hiltonworldwide3.hilton.com/en/best-price-guarantee/o...

http://www.marriott.com/online-hotel-booking.mi

https://www.ihg.com/hotels/us/en/customer-care/best-price-gu...