If the message "someone has collided with you" displays, I would hope that means the other person's front end was also relayed that message while the "you have collided with someone" message might mean only your front records the collision.
I really don't know how that part works but a mustang collided with my M4....no bombs.....I die and he flies off undamaged.
It depends on who sees what. From your point of view you saw a collision so you collided. from his point of view he didn't see a collision, so he flies off.
When you collide with someone you get the "you have collided with..." message
When someone collides with you (but you don't see a collision from your point of view) you will see a "so and so has collided with you" message, and NO message saying "you have collided with...." in which case the only damage you should take is from the round that the other guy hit you with BEFORE he collided.
Like I said before if you do NOT collide with any vehicles or planes on YOUR screen you won't get a "you have collided with...." message.