I guess theres tiny little incy wincy nano particles of other ores and materials in their makeup that is only extractable once you’ve scanned the material in question, marked where its situated so it doesnt get obliterated in mining the main goods.
That’s just my head canon until someone explains it better.
It works on lots of things, and make sure to scan the smaller variants as well, each one needs to be scanned at least once per planet. So if you see a stack of plutonium, scan all the little bits, medium bits and large chunks til it scans no more.
THis also applies with iron and a lot of other minerals. If you scan iron sometimes youll find it also contains copper, or iridium or gold or emeril. Its made things a little less grindy and a lot more exciting when you realise you dont have to spend the next thrity minutes hoping to find chrysonite.