I think Paulus must be right, It's a Northern thing ;)
That sarsen stuff though (with an 's' or a 'c'), It's fulla of natural cups. After seeing the things on Fyfield Down (Un-Beckensalled cups they are too) this summer, I gave West Kennet 's facade a good scouring for cups, there were a few that got me wondering. But then after a look at the holes in Wayland's Smithy, I wasn't so sure. Took pics of the Wayland's holes, but not of those on WK. Wish I'd taken the time to check the avenue.
So no help from me there really, other than to suggest the totally vague and unprovable idea that natural holes in earlier monuments could have inspired the production of artificial holes in later creations. Vice versa even.