The price of specific exchanges can vary quite significantly during dramatic price movements, because arbitrage can take somewhat longer in crypto markets than in most others.

There's also the fact that Coinbase has very high trading fees, and there's also the BTC and fiat fees.  Coinbase's price can easily vary by as much as all these fees combined (and the effort of doing the arbitrage).

Coinmarketcap is aggregated data, so it takes all major exchanges into account.