It's nothing to do with the exchange rate. The exchange rate is a result, not the root cause. The root cause is that Valve aren't interested in a flexible world-wide pricing policy due to either:
a) it's export on-line sales are relatively low so they're not too bothered losing relatively few sales. They'll make money from someone buying a boxed version instead.
b) they simply can't be arsed being competitive and are happy with making whatever they can out of people who are too lazy to shop around, aren't clued up on shopping around (unlikely seeing as they're savvy enough to actually use Steam), or simply don't mind paying a premium for a better delivery system and end result.
If Valve bring prices in line with the RRP then it'll still be more than most people can buy it for due to shops offering further discount. Valve may as well stay with the higher price and pull in extra revenue albeit with a smaller number of sales.
I'd be interested in seeing how much profit they actually make on a £38 game at the moment.