That's one of the problems with archaeoastronomy , was an observed or calculated phenomena indicated in the architecture by the builders ?. There is a convention that viewing is done from the centre of the circle (if you can find it ) but we don't know if viewing was ever done at all or if it was , where from , maybe it was the stone opposite the the tallest or recumbent in your case and not the centre . In the Midmar example the difference is about 10 degrees so it can't be both . Maybe it is all bollocks .