I reckon those ordinal numbers come from picking things up in some given rate. Say, picking every tenth of the cattle gave you `tenth' of the cattle. In the movie `The Tenth Man' you can see how the ordinal numbers are used in non-mathematical cases. `One over twenty one' is, I would say, a correct math term of 1/21. But `a twenty-first' will also pass for daily usage. Am I right?