Are you over 55? What you tapped into is probably an age-restricted fare. The very lowest fares are often reserved for seniors or students. Sometimes just clicking another button with the same fare will get rid of that restriction. Scroll over the "Details" to see the popup on age and other restrictions. If you can't find something at the same price, go up to the next lowest fare which is probably in an age-unrestricted booking class. If you click on the "All Fares" link a list will come up with a bit more detail.