Ah yes, but you did say "If I assume that I am 'Joe Average' ..."!!
You personally may well be paying more than that, that figure is arrived at by simply dividing the total tax take by the population, which is a pretty simplistic approach to take. For someone on £50k I would hazard a guess that it would be more like 50%, rising to perhaps 60% for someone earning over £65k.
Ouch, now my brain hurts too!!