Two theories I like, one is that Silbury is just a great spoil heap, dug out over the centuries to provide water - Loethar Respondek is the person who wrote the book on that one .... the other, 19th century, is that it was built like a pyramid, " that if you placed a pole on top of the hill the shadow would fall north on the level meadows, the daily gauge being about four feet “or almost, exactly that of the Great Pyramid”.
Think it was one of the earlier excavations, that a 'piece of wood' was found going down the centre, so you never know.....