grammatical number - Why do we use the article 'a' when referring to 100 items? But we don't use it when referring to any other plural count?

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:

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?