Cantjustread,

I am not a geologist but here is my crack at your question.

Percentiles are used in statistical analysis from what I know. So when the survey(s) used by mining companies return values which are not absolute I think they return as percentiles because the values fluxuate and the geologists are interested in the larger picture, not a very small area. The only absolute that I can think of that is reported as a concrete number is assaying.

I hope that answers your question.