Julian Cope presents Head Heritage

That's my point. A circle is easy enough to produce with fairly basic technology, so the fact that many are not circular and did not make use of that technology suggests that creating a true circle was unimportant. Which suggests that either the shape produced was deliberately not circular (for reasons that we may not know, if they don't relate to other extant features or something celestial that can be calculated), or the shape only needed to approximate a circle or at least look like one "by eye".