If you know elementary French, you know "dix" is actually the number for ten. But in France, when you quantify things in groups of say, 12, it's une douzaine—just like our dozen. But you can also quantify things in a group of 10: une dizaine.
Similar but different, the French do not use "two weeks" as a quantifier. If you take a two-week holiday, it's 15 days: quinze jours.