There really isn't any reason to offer any seats at the deeply discounted fare buckets for flights that are likely to sell out at higher prices. Families flying during President's week, who want to maximize their week, may have to pay more then they'd like. People on DIS don't want to hear this but high fares on peak flights is what allows airlines to offer deeply discounted fares on off-peak flights.
Some posters may have missed booking the "cheap" seats but it's possible cheap seats weren't offered.
Airlines don't follow this policy but I've read any fare that sells out in a few days never should have been offered. The airline could have sold the seats at a higher fare.