Coming in late here but to just expand things a little....
Because of the angle of the stone it suggests two things if indeed it is a 'carving'.
Either it was done prior to the stones erection (ooo missus!!), or, the stone was once a normal standing stone and upright to enable it to be done in situ.
To the best of my knowledge there has only been one writer/author that has not mentioned the off-centre stone leaning and that was the guy that first recorded it in the 16th century and brought it to the publics attention...
William Camden (Britannia 1586) described the stone circle as… “In a place called Biscaw Woune are nineteen stones in a circle, twelve feet from each other, and in the circle stands one much larger than the rest.
No mention of the lean you'll notice and let's face it, is it not the first thing you notice when in the circle? I've read oodles of reports since and never one where the leaning off-centre stone was not mentioned as being leaning. Stukeley reckoned it got its lean because of treasure seekers digging at its base, so we have to be careful in coming up with fanciful ideas another time as to why it was erected like that when in all probability it wasn't!