I've read that high-defence settlements (where the defence rating is higher than the combined food & water value) will not be attacked. Elsewhere, I have read that even high defence settlements can occasionally be attacked. Which is true? What is the maths/algorithm behind the chance of a settlement being attacked?

I think it would be a shame to create a very high defence settlement, armed to the teeth, if it were never going to be attacked so that you could witness those defences in action.