Here's why:
* Schwa is represented by the upside-down "e" symbol (ə) and is a very short, unstressed vowel sound.
* In "announce," the stress falls on the second syllable, "nounce."
* This means the first syllable, "an," is unstressed and has the schwa sound.
So, when you pronounce "announce," the "a" in "an" is a short, almost unnoticeable sound, similar to the "u" in "about" or the "e" in "the."