Welcome to the forum. In the year, there has been no change in the Abbys, the condition hasn't got any worse. I did check the other 3 cats today and the Pixie Bob hasn't any markings on her ears but they are heavier, thicker than the other cats.
The Manx cat, which is a grey tabby, has the same markings, but very small and not many.
My big male domestic short hair, which is white and black/grey tabby, with black ears, has a couple of very very small markings on one ear. I used a small light from underneath the ear to locate them.
Basically, I really have no idea what causes it. I will be talking to a neighbour who has 9 cats and ask her if she has any that are showing the same symptoms. If I find out anything, I'll add a note.
Animals are such agreeable Friends.
They ask no Questions. They pass no Criticisms.