I will answer this one logically at least.
It is because it gives them an excuse to prepare without making us suspicious. If they did all of this and stayed out of touch, we'd be on a hair trigger. [It's to keep our guard down..."it has to be a test..why would they warn us?"] We indeed, will NOT be prepared, since it is only a "test." It's just a chess game. The checkmate attack is disguised as something else. Do you play chess? If so, you'll understand.