This is a real thing. In Japan and other asian countries, braces are a fashion statement that flaunt your wealth. These are fake braces, usually bought at like a random shop and you glue them on yourself.
The reason they get absolutely straight is that the steel wire that connects the brackets is an I shape that will bend back into shape, taking the teeth with it. It is supposed to be a U shaped wire that is shaped like an actual straight set of teeth.