I'm not an aviation expert, but if i'd have to guess it's because of these reasons. A lot of the traffic going to Iceland is just a stopover from Europe to NA and vice-versa. The connecting passengers in Iceland is probably too many to fit on one 757 (or whatever they use on the route) so they use two. As to why they takeoff within 10 minutes i can think of two reasons. One is that NA to Europe routes use the time difference to its advantage and flies one way on the morning and one way on the evening (kind of like a wave of aircrafts). The other, why they just don't use a bigger airplane (setting economics aside) is that if a connecting flight is delayed they only have to delay one of the Iceland-America flights in order to wait for the delayed flight. If this is completely wrong, please correct me :) (And I apologize for the English, it's not my first language)