Underachievers have mental capabilities but lack emotional hardiness. They can't stand the hurly-burly of competitive achievement. So they don't quite get the degree, or if they do, get a lesser job. All in all, they don't get the objective status of their emotionally-hardier mental peers. Different things attract different people to Mensa, but for many the big attraction is that it provides a public badge of their mental abilities, a badge they can get just by passing a test, something they do easily without anxiety.
Noting some of the comments on this thread about people in Mensa leaning more libertarian, maybe they could be viewed as underachievers because they don't let others set their standards for them.