How come there's 'an' while there clearly should be 'the'?

English as a Second Language English Teacher Explanations (ESL Tests)
1

English Grammar Tests, Elementary Level

ESL/EFL Test #180 [color=blue]“Basic Grammar Exercises (1)”, question 10

That is … exciting movie.

(a) the
(b) an
(c) a
(d) much

English Grammar Tests, Elementary Level

ESL/EFL Test #180 [color=blue]“Basic Grammar Exercises (1)”, answer 10

That is an exciting movie.

Correct answer: (b) an

Your answer was: [color=red]incorrect
That is the exciting movie.
[size=200]_________________________[/size]

Really, I can’t get it how come there’s “an” while there clearly should be “the”?!
Wiki says: “A definite article indicates that its noun is a particular one (or ones) identifiable to the listener. It may be the same thing that the speaker has already mentioned, or it may be something uniquely specified.”
It’s evidently that the speaker knows about the movie & has already received certain amount of strong impression about it. So, shall it be ‘the’ after all???
Or do I miss something?
May be an indefinite article here means: This is one of the exciting movies I had seen in my life?

2

You’ve come close to the explanation with your final line, Gheon.
It is one of many exciting movies, so you need to use ‘an’.
If you use ‘the’ then you are indicating that this is the only exciting movie in existence.

“That is an interesting book.”
“That is the book I found most interesting.”