Fares are almost always imbalanced. We've had various examples crop up in this forum and the main explanation is 'What the market will take' If currency is trading high that might encourage those people to travel as they can buy more once they get to the destination, so you can up the price of the fare. Or it might be the reverse. You may be sure that the airlines know what they can get, because that's their business.
However, in this forum, we have found the answer is usually a shrug of the shoulders. You look at the fares and you decide which one to book.
There are other factors and you might want to look at the same dates for American, Delta, BA, Virgin and the others via various multi-carrier search sites. Also, consider if you have the funds in the UK and are booking from an I.P. adress in the UK not the USA? Also, if the traveller is young, they might not mind a 'dog leg' via Dublin, Iceland, Toronto or Boston etc. money can be saved.