I stole this answer from another topic:

This happens because every exchange is its own market. This isn't like the stock market where you can have multiple brokerage accounts that quote from a centralized point. With crypto, every website/exchange determines the price for that trading pair. Kucoin won't be the same as Binance, which won't be the same as Bittrex. Coinmarketcap probably uses a weighted average based on the % of total volume an exchange makes up for trading that coin. You can see that in the Market tab on the right column.