How to withdraw money from cash point at the genuine conversion rate? (United Kingdom)

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i hope this question makes sense.
when withdrawing from a cashpoint here in England into pounds sterling, there seems to be quite a large difference in how much should actually be coming out of the machine. for example i withdrew 500 POUNDS from the cashpoint last month, therefore i should see a transaction of around 715 DOLLARS on my payoneer account (including the fee), however i have actually been charged 782 DOLLARS! this is not the correct conversion rate, does anybody understand how this happens and if there is a way to stop it from happening?