True, I didn't look at it that way. It seems more likely that that's correct -- "Why those exact five?" -- but why would Quirrell find it so amusing?

edit: Maybe Voldemort has already hidden his Horcruxes in just those manners -- we already suspect that he launched one into space. In that case the riddle may be -- given that Harry and Voldemort think in precisely the same way, how can Voldemort think of a hiding place that Harry wouldn't think of himself?

edit2: It's out of character for them to come naturally to Harry, but not to Voldemort. Voldemort is into that kind of superstitious ambiance -- e.g. he wanted precisely 7 Horcruxes, because it's a lucky number. Harry is part Voldemort, so that's why they slipped into his subconscious.

*shrug* maybe I'm grasping at straws.