I was listening to a podcast earlier ("Very Bad Wizards" - always a pleasure, excellent work guys, thanks ; ) about humour in general... the types of humour, what's funny or not, when things are funny. It was an enlightening and fun experience, so if you want to listen for yourself, you can find it here.
But this is something that I'd been thinking about since decades. Black humour, nonsensical humour, slapstick humour, what do all these have in common?
I do know that when the brain is tracking 'movement', depending on the 'anticipation level', our subconscious will be trying to 'predict' what will happen next. I think this is the 'link' between humour types: most all types of humour 'break' from the pattern that we'd normally expect.
Whether it is humour or not would (I guess) depend on the circumstances, but I think it comes down to our relation with the source of the humour: in most cases I can think of, it is a relaxed state of trust. It could be another person, a television... and add to this the idea (IMHO) that our decision-making consciousness is almost a persona in itself (that can be trusted/mistrusted by our subconscious). So, from a position of trust, our senses get a description or circumstance that breaks from 'the predicted', yet, if the result is inoffensive in nature, we may even see sense in it. A sort of "I wasn't expecting that, but it fits." The "ha ha ha (how wrong I was to think that way)" may be just... sociological conditioning, an expression of a... "you got me"?
Going on to the part about "humour we don't find humorous at all", we may simply just be switching off the 'prediction/anticipation' brain function because we simply aren't interested in knowing what happens next, and that would also cancel any further reaction.
Just my two cents on a (still) mysterious subject.