Not sure where coinmarketcap is getting its data. But exchange platform differs from each other, I think every platform has its different pricing of the coin.
They update their data's on real time but sometimes their updating is not accurate since their are some coins who are having a fast trades and fast pump and dump scene and coinmarketcap cannot update it faster since I think their bots or whatever they use have certain time to give some price updates.
Coinmarketcap gets the data from not one exchange but from many exchanges and averages it i believe. Thats why the difference. For everycoin, if u click markets tab it will show u what are all the markets its collecting data from.
Maybe that's one of the reason why we can see them slow upon updating on the coins price.