This is always confusing to me.
I've come to the conclusion that there is an optical allusion that is throwing off my ability to judge altitude. I have to remember, I'm looking at an plane flying over a circular object that has a gravitational pull. This is keeping my plane, and their plane, level, at least to each others location.
If flat, I would see the plane at the same altitude I was at, and I would see the plane level, side on or head on. Flying over a curve, I am flying "level", the other planes are "level", at least relative to their exact location. They are off in the distance, level over a curve, so I am actually seeing the underside of their planes. Place your hands flat in front of your eyes. Now spread them apart, but imagine they are over a curve. Same height, but you're looking at the underside of your hands now. In the air, you instinctively think if I can see their landing gear, I'm under them.
But, my lack of scientific edumacation suggests I'm just kinda winging it here. Looks like they are higher, looks are deceiving. There are a whole lot of missing elements that help the brain adjust to altitudes, position and such out in the real world that simply can't be duplicated on a monitor, so the effect seems more pronounced on screen.
IMHO, YMMV, and other required disclaimers suggesting I have a clue.