For simplicity's sake, we need to display a single value of a specific coin at any given time. We currently do this by using an aggregate value across multiple exchanges, which can vary from the specific price that you see on any particular exchange. We are working on improving this by moving to per-exchange pricing.
Our real time pricing is set to display token prices based on a single exchange per coin. This is a trade-off to enable us to show you live updated prices and an area we're working on improving.