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?

Question

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
• 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?

Answer 1

a is replacing one.

That is, in the following pairings, both options are legitimate:

I saw one hundred [and] forty-seven birds today.¹
I saw a hundred [and] forty-seven birds today.¹

I earned one million dollars.
I earned a million dollars.

Preferences for one or the other may vary. To me, using one sounds more precise than a, so that would influence my usage: in the first pairing, "one" sounds better to me, while "a" sounds better in the second, unless I was trying to emphasize that I earned exactly one million dollars.

Regardless, either option is legitimate because a is a synonym for one. The reason your other examples don't work is that they don't start with one.

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1. The use of "and" in the first pairing above varies based on region (US vs. UK) and speaker preference. See How do you correctly say large numbers?

Answer 2

"Hundred" is not plural! It is singular. You have one hundred. Hence the "a" ( or "one").

Consider:

I saw a hundred birds today.
I saw two hundred birds today.
I saw three hundred birds today.

Also:

I saw a dozen birds today.
I saw twelve birds today.