Would it be possible to have imperfect sensors?
So that that little flotilla you planned to ambush is a little bit more than you expected?
Or for example if you see the enemy launching fightes you can't tell if it's 4 or 5 squadrons?
naturally if it works for computer it should work for you.
the difference shouldn't be that large, say 5-30% of the fleet, and not always? percentage could be influenced by stellar phenomena or objects, proximity to stars...