Yes, that is a great thread. Herbie had it 99.9% right! We argued back and forth about many concepts, but mostly the failsafe (or lack of) nature of a siphon system. Without the emergency drain, you will have a flood sooner or later. Setting up a siphon and open channel drain without a 3rd standpipe is akin to running on the ragged edge.
The actual water level in the overlfow box does change depending on many variables. The system is very self tuning but can get out of whack. Obstructions in the valved area can occur as the result of algea breaking free. Barometric pressure and slime coats can cause differences in head pressure and friction...
Thanks for posting the link to that old thread, it is full of great iformation, even if some of us have added to our knowledge since that time.