That's interesting, isn't it? I think this proportionality is related to a quirk of human perception. If you are shown briefly a set of objects the average person is able (at a glance) to quickly count those objects for a low value whose threshold is around six or seven. Thus when the number goes any higher the individual will respond that there were "less than ten" or "fewer than ten". Of course because humans have standardized on base 10 as there counting system then ten will get bumped up in any distribution but to predict that it would occur between six and seven (the number of man and divinity no less) would be difficult.

What about 11 to 20? Hmm, it (roughly) goes: twenty, twelve, fifteen, eleven, eighteen, fourteen, sixteen, thirteen, seventeen, nineteen! Just what I expected :)

1,2,3,one,two,three

http://ngrams.googlelabs.com/graph?content=1%2C2%2C3%2Cone%2...

for a short period of time in the 80s and 90s, 1 was more popular than one, where from the 1700s through 1950s, 1 was half as popular as one ...

Also interesting, the 1700s were not kind to the numbers "six" or "seven":

http://ngrams.googlelabs.com/graph?content=one%2Ctwo%2Cthree...

perfect example of http://en.wikipedia.org/wiki/Benford%27s_law