This is a solubility test that shows lockout. You must be referring to the Mehlich 3 test, I F'ing hate that test. It's tits on a boar, don't tell you anything...Very interesting. What potting soil was this specifically? They must have actually went to trouble to get it below 0.5 ppm! Lol
I say that because it's pretty damn hard to find anything in nature that contains less than half a part per million of Mn anywhere - certainly not a leaf or any other organic material.