I learned of them through this forum;
@poketrade Pokémon ex series serial code variants! Let’s make a list!
@tonysandlin Request ex series codes - Full list of codes added!
I seem to recall @Quuador knowing a bit about them as well, but I don’t think anyone was ever able to confirm their purpose. Perhaps it’s got something to do with different sheet layouts, or different printing facilities, or different positions of different copies of the same cards within a sheet, or maybe it was all just a funny joke they played on fools like us - it’s unlikely we’ll ever know.