I think that the reason for this is quite simple. Poor graduate students are often so excited about going to a meeting that they get their slides ready before the meeting and actually rehearse, often compulsively, before their peers, their mentors, and before their long suffering moms and siblings. They cut and prune their text to make sure that the talk ends within the allotted 12 minutes, and that there is time for questions. If you add to this the fact that they are often nervous and talk faster than they have done in the conference rooms of their home departments, they always end well before the bell goes off.
Most P.I.s seem to harbor the unjustified conviction that they can give any talk, anywhere extemporaneously. They always misplace a critical slide, and tell you that they left it in the airport in Zurich (just so you know that they ...
