I found this reference that seems to explain it pretty well:
http://www.nols.edu/resources/research/pdfs/lightningsafetycavers.pdf

In summary, near the *entrance* to a cave may be more dangerous. Also, standing under a rock ledge may be more dangerous because you are in effect standing next to a tall object (the outcropping/cliff/whatever) that may be likely to be struck.

However, if my only other choice is to stand on top of the same outcropping, I'd prefer to stand underneath instead.