That's the only explanation I can think of for this odd occurence.
I built this retaining wall on the back side of my property to prevent additional erosion from the slope. The previous owners had essentially let this part of the land (and much of the rest) fall to ruin. When I built the wall two years ago, the ground underneath was quite solid, and I laced the first course of brick nearly flush with the surface. I never would have expected to find that last course nearly flush with the surface as well.