Unequal exchange rates in purchases

Am I the only one who notices that the costs of packs are not equal across currencies?

After living in the US and the UK I can confidently say that the packages do not follow the exchange rates. A package that costs $20 does not cost the same in pounds £15 at the current exchange rate (0.75), so why in the store does it cost £18? It’s just not worth it to live in the UK and pay for packages in pounds since you’re paying more than you would in dollars!