It could be a couple of things, Dee Sa... Firstly, the large difference in the exchange rate. The Pound Sterling is worth a lot more than our sad sack dollar. Additionally, even here, the difference between a fare purchased three weeks or a month in advance is always far less than one purchased just a few days in advance.
Lastly, there a small number of "niche" airlines, such as Allegiant (
http://www.allegiantair.com/ ) that operate on a semi non-scheduled basis. They're a good, safe airline, but they operate from point-to-point rather than hub-and-spoke and they only fly on, say 4 or 5 days per week rather than daily.
I'm glad you enjoyed your American sojurn! Come again...