From: Winnipeg, MB
I also get this at times when unloading at small ports, even got it once at Auckland when it was I think size 7. I figure the desingers must have put in something to account for faulty or insufficient unloading equipment
At small ports, larger pieces of equipment are transferred to shore or ship by lighters. A lot of crane work and rigging, or even block and tackle work is involved.
In addition to equipment failures, the experience of the ship's crew and units ashore might play a part in the calculations. Poor experience does not go well with rigging and hoisting/swinging a large, awkward and heavy item. I think the abstraction of a small port into odds of failure in offloading items with large load cost probably includes these sorts of considerations.
No matter how bad a situation is, you can always make it worse. - Chris Hadfield : An Astronaut's Guide To Life On Earth