**Edit Note**:

This question has been linked to these questions about using an article before adjectives modifying numbers. This question here has nothing to do with adjectives at all:

* [Indefinite articles used with plural nouns: It was AN amazing TWO DAYS](https://english.stackexchange.com/questions/252173/indefinite-articles-used-with-plural-nouns-it-was-an-amazing-two-days)
* [Indefinite article in the "An [adjective] [number] [plural noun]" construction](https://english.stackexchange.com/questions/9933/indefinite-article-in-the-an-adjective-number-plural-noun-construction)

**The question**

Why do we use the article 'a' when referring to 100, but not when referring to any other plural counting?

Here's an example:

> 
> I saw a hundred birds today.
> 
> 
> 

But when talking about other count that 100, we don't use 'a' because it's plural:

> 
> * I saw ninety-nine birds today.
> * I saw two birds today.
> * I saw two thousand birds.
> * I saw seventy-five birds.
> 
> 
> 

Is there a grammatical or historical explanation?