The long hundred, a unit of count = 120, appears to have arisen out of an ancient Germanic way of counting, one which is echoed in modern English. The “teen” suffix, as in four-teen, six-teen, etc, is not applied to one plus ten or two plus ten (they aren't one-teen and two-teen). Eleven and twelve are treated the same way as the first ten numerals; the break is after twelve, not ten... Note that this is not a duodecimal numeric system.The use of the word in this manner lingered for a long time in England:
This reckoning of one hundred as six score still holds good (or did to my knowledge ten years ago) in Leighton Buzzard, Beds. If one ordered there 100 plants, for example, one received, and also had to pay for, 120: a hundred being always reckoned as six twenties. If one required simply 100, it was necessary to order five score.
So also here in Cardigan and around, taking eggs, for example, the dealer picking up three eggs in each hand, reckons that twenty times this makes one hundred.Emily M. Pritchard (writing on 21 July 1906).
Archaeologia Cambrensis. 6th series, vol. 6, page 352.
Learned from a recent podcast of No Such Thing As A Fish (thank you, elves).