<p>It is not obvious to me. Indeed, in other contexts, people who are angry about something (e.g. customer service, product quality) are more likely to tell other people than people who are pleased about it. So, if anything, the angry unemployed graduates may be more prone to responding.</p>
<p>Also, some schools do their surveys six months or a year after graduation, when getting graduates to respond (or finding them in the first place) may be more difficult.</p>