I have recently bought and sold some USA shares and have been unable to understand the way foreign rate conversion rate is applied. When I buy USA shares, the conversion rate is somewhat in line with whatever the standard market buying rate at that particular point is but when I sell them it is way higher than the market selling rate. Even with the 0.5% IG fee added, it does not make sense. For example on 09/06/21 I bought some USA shares and was charged £1= $1.408 conversion rate which was fine but when I sold the same shares after an hour, I was charged £1 = $1.460. The market spread for whole day was between $1.40 and $1.41. Any explanations ??