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.
I do not think that lazy because ANN means that when an update or new information is announced for announcement it is first communicated through ANN so that everyone can be aware of this information Able to get.
From what i know, ANNOUNCEMENT is just abbreviated as ANN because the title space has a limited amount of characters one has to use. Ir has nothung to do with Laziness.