Cost of Airline Tickets ???

While on my flight (which by the way was a 11th hour booking) one thing irked me...why is that when u book a flight early you get a better deal(often rock bottom prices) whereas a last minute flight is costly (nearly Double, like the one i have shelled out now)....Shouldn't all the seats cost the same irrespective of when they are booked?? Quite not able to come up with plausible explanation for this....The search is on...