Cheaper doesn't always tell you "least busy", especially when planning ahead. I'll have the misery of flying the day before Christmas this year and the flight itself was peanuts but I'm not looking forward to the airport situation.

A lot more goes into flight pricing than the available space. They'll look at their data and sometimes hold out on high prices, hoping for someone to bite on 1 ticket they desperately need vs trying to sell 5 tickets at 25% of their current price.

Source: friend works in pricing for a major airline