Bioequivalence and Bioavailability Forum

...and all this naturally leads to the next big question, slightly OT but to me a much more important related matter

Let us just say we work with a producer of ref standards located in... say we pick a completely random place... Toronto, Ontario, Canada.
Said producer renders CoA's where the purity of the ref material is given with two significant digits. Most often 99% or 98%. If you are very lucky you'll get something like 98.7% (three significant digits) but that's an exception and you cannot request a CoA with a given number of digits.
Now, how (or rather why) the [expletive] does one report concentrations with 5 significant digits from std curves produced from such standards? And how (why), in the grand scheme of things, does one meaningfully and reasonably get from such concentrations where two digits are the most you can squeeze out, to confidence intervals reported with four significant digits on the Western side of 100% and five digits on the Eastern side?

Does anyone, anywhere have anything better than "those are the rules" and "we just follow the standards"?