Some forex brokers are on the other side of the transaction. By this I mean that when you enter into a transaction, is your broker also the counterparty to the transaction. If this is the case then, anomalies (lets be euphamistic and call them anomalies) occur which ofter cost the investor. For example some brokers are reputed to engage in stop loss harvesting where a sudden dip or spike in the quoted rate will cause the stop loss to close the transaction and the investor to lose money. The broker knows what price fluctuations will affect his profitability so you take a risk if the broker is thecounterparty to the transaction and you should always check on when choosing a forex broker or any other broker for that matter (for example CFD or share trading)
Don’t be fooled by the size or apparent reputation of the broker, but always make these checks as many big name city brokers can have a conflict of interest in setting rates.