It works that way on a lot of routes.
London-NYC-London is typically £430 (about $660), whereas NYC-London-NYC is normally well over $800. Same company, same flights, completely different price - just on the basis of the origin city. And it's the same no matter what airline it is - the fare levels on AA/British/Delta/Virgin are usually identical.
What's more, but the fare rules are different too. The change penalty when originating in London is £100, just over 150 USD; out of NY it's $275. Why? Because. Then again, advance purchase requirement in UK for the cheapest fares is 7 days or more, these days out of NYC it's zero. 2 years ago it was the other way around. Why? Again - because.
And no, it's not the taxes, which are clearly marked as such; the fares themselves, down to rules, are totally different.
The airlines' fares don't usually have rhyme or reason as far as costs are concerned. It's about what they calculate will better fill their planes and make them more money, based on their understanding of the market and whatnot. Often it simply comes down to what they decide a given market is willing to pay.
And by the way, while prices here are indeed cheaper from Europe, the same is often not true of the US. Flights from here to NYC are often cheaper on AA than vice versa.