Unlike stocks or futures the spot Forex market does not have any centralized exchange or clearinghouse. Alternatively, each broker acts as its own exchange and the broker effectively becomes the market maker.
This, in fact, provides an opportunity to abuse on the part of the broker or worse. Because of the absence of a centralized exchange, we also see price variations from broker to broker.
When dealing with reputed brokers in well-regulated countries these differences will be small but you need to be well aware of this fact especially if your charting data provider is not the same as your broker, as this may lead to inconsistencies between the planned and actual execution of trades.