I got the impression that HD was trying to convey the fact that judging altitude accurately from the ground, especially when the aircraft are into the vapour-revealing levels, just using Mk1 eyeball is extremely difficult. I'm sure you, like me, will have seen two aircraft which appear to be at the same altitude, heading straight for each other at an angle of 90degs only to have them pass each other safely leaving a nice + shaped trail because they were at different levels.
