Certainly a possibility I agree. And one that would make it all too likely that they lost track of exactly where it was so that it is now off centre.

As Dean Merewether wrote -

"It could hardly be expected that these two small openings would be likely to find the primary grave under Silbury hill than two rat holes would be likely to come upon the ashes of a mouse placed under a mound 10 feet in diameter."

Considering they are exactly the same rat holes today that he was talking about then, he's still right I reckon.