The mistake you are actually making is to compare BTC-USDT pair with BTC-USD pair. It is always a possibility that two different exchanges will have two different prices as they are both two different platforms and this is how some people do arbitrage trading, as you can read more on that if you have no idea what that means.

The reason you are seeing such a huge gap is because one is a USD pair and the other is a USDT pair. On bittrex alone where the both markets are inexistence, there is always that huge gap between BTC/USD and BTC/USDT pair. I really do not know why but that is just the case.