The power of rust!
I've never notice this before - rust has "grown" between the bottom blade and the sole plate forcing the two apart between the screws!
I suspect that there was an air gap between the bottom blade and the sole plate. It only takes a minute amount of moisture and air to start the corrosion process going then the result is plain as day.
When a bolt head is exposed to seaspray the corosion will cause a large build up of scale that to form on the nut increasing its size considerably - until you knock it off you then see the degree of wastage that has occurred.
When I was inspecting cranes, steel quayside ladders or bridge structures where corrosion is often found an allowance of 10% wastage was the guide for assessing the structural integrity of component parts.