Forex is an OTC (Over-the-counter) market. It means: different dealers, different sell/buy prices .
Back test your strategy on the dealer you are using.
If the broker does not offer back testing or historical data, find another dealer.
In addition to back testing, monitor using a third-party system such as E-signal Forex data (50 dollar per month subscription) or some other real-time data provider weather your dealer gives you significantly different prices from the other dealers . If yes, change the dealer, he might be even cheating with the quotes you get.