H/T: Frontal Cortex.
Commenter Rachael asks some good questions about what might be going on here:
RE: the whole Boss/Employee thing, I'd have two (far too sciencey) questions:
1) Do the people actually find the joke funnier or do they just know they should laugh because it's the boss?
2) Is their mood upon entering the conversation likely to be different? Maybe the boss gets slightly annoyed whenever an employee stops them for questions/time. I think in general people are pleased to be addressed by a someone they perceive (for whatever reason) as higher up than they are.