Yeah I think they tried to mirror a humans reaction on this one. If you fought a battle where the enemy had 16 units in the pre-battle screen, but only 10 on the battlefield, you know that there are probably 6 hiding somewhere.

Now you could probably figure out the most likely spot for them to be. If you scout out to look, and the ambushers charge out, your scout is going to be surprised. You knew they were on the screen you just didn't know where.