That's weird,
I've found loads of ELW, AW, WW (I only map these) and you get even more if any planet is Terraformable (my highest was 8.5M Cr, although can't remember if that was for one planet or the whole system, prob whole system).
I go about 2000LY in my FC, to random exploration spots and do a 50LY sphere of all systems - get loads of undiscovered also at this distance.
I also do mining for Plat, Osm, Pall, if I find a Pristine Metallic ring while I'm there, which is better than mining for Tritium.