They are not! Sencha should be brewed with off-boiling water, anywhere from 85 - 95°c (which is still a higher temperature than low end sencha).
What accounts for this difference?
High-end sencha is steam processed at a lower temperature. Steam only gets to ~100°c depending on altitude and that is the highest temperature it is exposed to throughout its production.
High-end Chinese green tea is partially processed in a wok to denature the oxidizing enzymes. The saying for Chinese tea is that "tea remembers the heat of the wok", which is the pan frying process at upwards of ~200°c.
So all wok processed green tea can be brewed with hotter water than steam processed green tea.
