Re: subunit display - generally speaking, nations (the ones in the game) know the force structure and equipment list of their opponents. Tactical intel folks (Div and lower echelons) have a good idea of what units are operating in their sector and how they operate.
My mind goes in the direction of the enemy OB briefed is not what the player encounters. This could be from a mis-identified enemy unit (as provided by higher HQ in the enemy situation) of the unexpected appearance (surprise!) of a unit not seen in sector up to this point.
It's not really a matter of "Oh, we thought they had T72s, not T80s. Sorry about that." But, which Bde of an OMG moves into your sector? Yes, that could be a situation where what you meet is not what you were told to expect. Another that could reasonably happen is an airborne or air assault that caught one nation by surprise.
Obviously, this kind of feature is not trivial to add if we go the path of this level of situational fidelity. In a real command post, the intel types have a list they update of expected enemy units, confirmed enemy units, and reported kills on those units. That might be a cool thing to have in the game.