I asked my GP about it once and he admitted that the entire thing was nonsense. Note only does it not consider the person's size or general health and fitness, but the limit is the point at which they believe you can begin to detect any impact at all. If that impact is tiny, you could extrapolate out to 60 to 70 units a week and find that the impact is still miniscule.
Admittedly, I say this on the back of a week where I achieved an historic level of sobriety only to discover that I totaled 36 units.