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
- Indefinite article in the "An [adjective] [number] [plural noun]" construction
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?