Does FW's definition of a Cairn has anything to do with this?

From Megalithomania:
A cairn is a large pile of stones, quite often (but not always) containing a burial. Sometimes they have a kerb around the base.
Most cairns are hemi-spherical (like half a football), but the piles of stones used to cover wedge tombs, court tombs and portal tombs are also called cairns. When associated with these types of monument they are not always round, but sometimes rectangular or trapezoidal.

There you are: "half a football"; the other half is underground of course.

Lets wait for a "Rugby Pyramid".

Cheers

Jan

PS: I'm sure you'll find that explanation, Hob!