Each exchange is their own little micro market of buys and sells (bids and asks). Back when the market cap was way smaller they would vary even more than now. You could sometimes buy on one, move the coin, sell on the other and pocket the spread with a profit even after fees. It is this kind of behavior that brings them in line with one another and generalizes the price across the exchanges. But still, each is a micromarket in the macromarket and there’ll be minor differences due to local activity and exchange policy choices.