The problem, especially for heads resigning is that the new school know that they've appointed him before the current school know he's resigned, so if someone from the new school leaks the news there isn't much that the current school can do other than confirm it as soon as possible (which it looks as if they have done).
It is extremely excruciating for governors at the current school, if the new school talks about their new head, and he hasn't actually officially tendered his resignation yet.