Many businesses have distinct areas of work, where one part of their operations currently has to be closed down but another part can continue to operate normally. For example, Currys PC World has had to close all its stores, so its perfectly logical thatall its in-store staff should be furloughed on 80% pay. However the company can still operate online, so its warehouse staff can continue to work as normal, on 100% pay.
Other firms might not have such distinctive job definitions but still find that there's currently only enough work for just part of their regular workforce. So it's perfectly reasonable (and lawful) for them to retain some staff, doing their normal work, on 100% pay while placing the others on furlough at 80% pay.