Pay in dollars if possible

Strange title for a post in this blog but this is interesting.
I've used Last.Fm for a while but only when it was free. So today I decided to subscribe (well, just for a month to try it out).
Right now I'm in a country where Euros are used and the price is give in USD, GBP and Euros. Guess what though, the price is always 3 of whatever currency unit you select.
So, even though the Euro price was €3, I decided to pay in USD, so it was $3.
No problem, and my account was debited €2.18 which represents a 27% discount.
Presumably for UK-based people, a discount of close to 40% could be obtained by paying in Dollars rather than in GBP.
I really don't understand the logic behind this.