I think the underlying premise is that most bands, including spork, have things in common with Spinal Tap. Seeing the movie after having been through those situations, I can't help but see the film in a different light. I didn't get it at first either, but then the picture of Jeanine always makes me crack up.I still don't get it.
I think it was Steven Tyler who said that Spinal Tap really wasn't funny at all because it was too real. Although I'm sure some people reading this wish we'd had an exploding percussionist, if I remember correctly.