Update: For anyone who is also confused by this, I have learned the answer. Da is never used when discussing numerals, e.g. cent, tridek, dekono.

No; in English, they are synonymous, and in Esperanto, we don't say "unu dek" or "unu cent", and usually not "unu mil" either: http://bertilow.com/pmeg/gramatiko/nombroj/vortetoj/formoj.html

What do you mean "as if they were nouns"?

Fractions are nouns.

You can say "a fifth", "the third", "my quarter of the cake", "the blue half and the green half" just as you can say "a book; the book; my book; the blue book and the green book".