Two aircraft heading towards each other in each pilots one o'clock. Not quite likely to collide but too close for comfort.
I wonder what happens in practice?
I have a horrible feeling that, in many cases, neither pilot sees anything and (usually) they do not collide. Neither would be the wiser after the event, so who is to say how often this happens.
Otherwise it is entirely reasonable that one pilot sees the problem first, and takes evasive action, to the right or left, depending on how "one o'clock" it was. Now he is changing shape, showing more wing, and a moving target (sorry, that's definitely the wrong word) in the other's windscreen, and is now much more likely to be seen, and the second pilot can evade in the other direction.
I don't have that many hours, but I have been close to other aircraft at least 3-4 times, that I know of, excluding cases when somebody like Farnborough Radar told me to look out. I don't think the other aircraft saw me in any of those cases. Doubtless there were cases where I was the blind one - I wouldn't know.