There are examples of it happening in other countries though. Zimbabwe and Argentina come to mind for some crazy inflation and stock numbers.
Asset pricing is not continuous. A single trade can take pricing from -18% to -X%, with X having any value between infinity and -100.
The breaker triggers at -20%. If the market crosses at -18% and then trades -25%, that trade will cross and then trip the breaker.
There are examples of it happening in other countries though. Zimbabwe and Argentina come to mind for some crazy inflation and stock numbers.