I'm tracking BTC rates and they are all different??
Coindesk is a news site and reports an average price among different exchanges, i don't remember the details but you can read it on their site i think it was called BPI
cointelegraph is also news site and i have never bothered checking it because it is pretty bad, but it also takes the price from somewhere similar to coindesk and reports it.
poloniex doesn't even have a USD market, it instead has USDT which is an altcoin with the same price of USD and since it is a different market in many aspects the price is usually different too.
Kraken is a bitcoin/fiat exchange and that price is live and reliable among these. but if i am not mistaken it doesn't have USD but only has EUR markets (have to double check)
and as for exchange prices (bitfinex, bitstamp,...) differences as other mentioned since they are different and not connected to each other the prices become different from time to time but since they are all trading same thing bitcoin versus fiat the price balances out fast enough.