While Great Whites are known for their powerful jaws and impressive teeth, they don't actually take a perfect circle out of seals. The "cookie-cutter" bite shape is more of a crescent or semi-circular shape, with a distinct, clean edge. This is a result of the way their teeth are arranged and how they attack.
The shape of the bite is also not always the same, as it depends on the size of the seal and the way the shark attacks.