Those numbers are the distribution of materials that you'll find in random meteorites and outcrops. You'll get 3 of a material in each chunk but when the planet says "Iron 23.4%, Sulphur 19.2%,...Polonium 0.7%" that just means you'll find a lot of iron and sulphur for every polonium chunk you find. The total amount on a planet is effectively limitless for every material as the shootable rocks respawn constantly - a few times I've picked up a metallic meteorite on the scanner, driven towards it and found it right in the wheel tracks of when I went past that spot 10 minutes before when it wasn't there.
Volcanic and biological sites follow slightly different rules, I'm not sure if a 0.5% vs 1.5% of the rarest material effects the number of needle crystals or the biological equivalent whose name escapes me that you find at each site. Once you clear a site it will respawn its materials a couple of weeks later.