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

I wasn't sure how best to phrase the title of this question.

I'm interested in constructions of the following form:

An estimated 50 people died in the bombing.

'An estimated' could be substituted with adverbs like 'approximately' or 'about', so it seems that prepending the indefinite article to certain words produces an adverb.

My question is why the indefinite article should function in this way.

For one thing, the use of 'a[n]' when the following noun is generally plural seems odd.

There are quite a few words that fit this pattern:

• An additional ...
• An approximate ...
• An estimated ...
• An extra ...
• A good ...
• A huge ...
• A mere ...
• A possible ...
• A record ...
• A scant ...
• A whopping ...

Dictionary.com's fifth definition of a is:

5. indefinitely or nonspecifically (used with adjectives expressing number): a great many years; a few stars.

This usage of a is standard and often seen in this type of context.

Perhaps similar to Dante's answer: I think we use a article because we are referring to the "estimate" (or whatever), rather than to the "people", and there is only one estimate.

If I said, "I have prepared an estimate of $1000," I think there would be no question that "estimate" is singular and so calls for an article. The question is, when we say "estimated", as in, "The population will grow by an estimated 1000 people", estimated is beig used as an adjective, but what does it modify? "People"? But the people aren't estimated, it's the NUMBER of people that is estimated. When someone walks in a room, you might say, "Oh, he's a very tall person", but you wouldn't say, "Oh, he's an estimated person"! So in that sense, I think Dante is right that there's an assumed noun omitted from the sentence of "number" or some such word.