It actually depends on the market. Every minute or seconds a lot of people trade on the market that's why bitcoin's price isn't stable or not the same in every moment of bitcoin's currency.
Which market? Each exchange site has a different volume, depending on a users who use the exchange. Maybe that's what makes the price of bitcoin different from one exchange and the other. Whereas if you look at the coinmarketcap the amount of the price is accumulated from the various exchanges, or the average of all exchange cryptocurrency.