High tidal range at St Peter Port

We were astonished by the tidal range on Guernsey. Admittedly, we arrived at full moon so there was a spring tide. But look at the height of the poles that allow the floating jetty to move up and down. It's something like 9m. They said the 3rd biggest in the world. Not true, but in the top fifty. See http://www.co-ops.nos.noaa.gov/faq2.html#26 I have read that they are all close to big bodies of water. True of Canada and Chile, but why not the coasts of Africa? And why the Bristol Channel but not the Outer Hebrides or Norway? Is it just about the shape of the coastal shelf?