I'm not sure it's actually a pontil mark on the base?,... it looks to me like some sort of water deposit, as there's also a run or drip, of the white coming from the centre.....
...however, if it is a pontil mark on the base, I'd agree that it's highly unusual, and something I've never seen on this shape before. I'd be surprised to hear that other makers had made the same shape, colour and size of vase, as Caithness, though it is a possibility of course.
Just to be certain, I'll email the seller to ask whether it is a pontil mark, or a stain. I'll let you know if I get a reply.