@fromnowhere I have been wondering about this point as well. Any consultant, working via a Personal Service Company, in addition to other more traditional business owners will likely post relatively small "Salaries" but they may still earn quite large amounts each year. It would be good to understand whether the statistics quoted include or exclude these individuals.
I personally suspect that the figures are based purely on amounts earned via Salaries and taxed under PAYE. Many higher "earners" will not fall into this pool.
(I take the point from @BarbaraofSeville that there will also be other non-salaried workers who earn relatively low amounts as e.g. drivers. It would be equally valuable to know whether including this group would shift the 5% mark to a higher or lower annual income.)