Why use higher ISO when using a tripod and the object is static?

Sometimes when I see a photo such as this one, I wonder why the photographer chose such a high ISO (640), despite using a tripod with a static subject.

As far as I know, a lower ISO means less noise, and vice versa. When shooting in low light with a tripod, it should be possible to just increase the shutter time. I believe the photo can be taken at a lower ISO without any major problems. So is there any other reason?

[EDIT]: For those who can not open the link, here is the photo:
_{(source: amateurphotographer.co.uk)}

Equipment and settings: Canon EOS 5D, 24-105mm, 1/6sec at f/22, ISO 640, tripod, ND grad filter