It's a simplification over the real world, where prices are based not only on where you're going and what class you want, but when you buy the tickets (someone who walks up to the counter the day of the flight is going to pay much more than someone who reserved at least three weeks in advance, etc). It's so that you don't have to worry about putting something temporal in the equation for calculating the prices.
But you're definitely thinking of it in the right direction.