That is - one ore patch covered by another during map generation wiping out the former. From another thread I saw that game actually supports many overlapping ore patches, i.e. a tile is allowed to keep both ore type (but you have to use console commands for that, even map editor won't allow it). Like here:
MultiOre.jpg (610.08 KiB) Viewed 1537 times
Now the question - why forbid such patches in map generator, so instead of wiping we get tiles with both iron and coal for example? Such combined ore patches can be fed directly into smelting line in early game and not so much later you may filter it out with smart inserters.
It would certainly be interesting as a map generation option, but that would be really, really annoying to deal with in the early game without smart inserters to do sorting.
Edit: I guess you can technically "filter" with storage boxes with the slots limited. But that's really finicky.
If they changed smelters to be lockable to a single recipe it might be tolerable, since then you could split/sort resources that way.
Smart inserters are just 30 bottles away from automation research - i.e. something you get for granted by the time you put mass electric drills.
edit: another issue of similar type is when supposedly large ore patch is mostly covered by water, especially in starting area. I'd vote that such patches are either removed or enforce land beneath if that looks organic.