If you get a message in orange that says "You have collided," even if it seems like it was his fault, your computer has seen an aircraft "enter" your aircraft, and you receive damage.
If you get a white message that say, for example, "OOZ662 has collided with you" and no orange message to accompany it, your computer probably saw me fly right past you. On my side, I plowed right through you. If I didn't fire my guns, you take no damage in this situation.
On rare occasions, the internet lines up enough that both people get both messages, and both take damage.
The game does nothing to determine whose fault it was or what order things happened in. That is a myth.