ANN and announcement both are same. People are very lazy so they have trouble writing the whole word. They have figured out a short form of it. Now everyone uses it.
That's not the truth. Not everyone creating announcement topics are lazy. When making a topic, there's a maximum amount of characters allowed so it's all about keeping things simple and precise. Afterwards it became a trend and gets things done.
@OP ANN == Announcement But you should consider using the former in your topics. No one is interested in seeing the full spelling + it'll save you more characters.