Even in Forex dude, rates of EUR/USD or USD/JPY or any pairs you pick is very different to hundreds of brokers online. Same as in crypto , there are thousand of traders online. Some of them are playing arbitrage. For example if BTC is cheaper in Coinbase. Then you can Buy a lot of Bitcoin in Coinbase, and withdraw it. Send it to GDAX to sell it in a MUCH HIGHER price.

#PROFIT

Rinse and repeat.

#PROFIT

One more thing prices varies on users, some has a very huge money. A huge buy can produce pump or much higher price in ones exchange.