Bittrex is not accepting Fiat, that is why sometimes rates can differ, because people do not have this much BTC to buy, for example.

Also, of course, prices may be slightly differ depending on people who are trading there. The persons who usually equalise the prices are called arbitragers. They see the price difference, and quickly buy on one market and sell on another. When there are many people doing this, the two prices equalise. This happens on non-crypto markets too.