Say you're driving on a major freeway. Night has fallen and there's no traffic, but it's around 8pm, so it's not super late or anything. You're driving 75 mph, which is over the speed limit but definitely not an uncommon speed for this freeway. While in the left lane, you come up on a car that is not going faster than traffic and not passing anyone else. You pass them on the right and get back in the left lane with plenty of room (like you don't even get close enough to read their license plate) between you. The driver flashes their brights at you.
Why did they do that? Were they calling you an asshole for speeding? Or did they think you were too close? Or other??
I've heard sometimes people will flash brights to warn of cops ahead but we were going in the same direction, so that doesn't make any sense. Besides, there was another driver who flew past even me, but he was in one of the far right lanes. So if there was a cop and the brights-flasher somehow saw him in the dark, he probably would have gotten that guy.