Peekay4 it may be a difficult bit of maths but a zero alcohol reading IS 0.00%
No you dont necessarily know if you have an underlying medical condition, and proving it is hard if you do. And as others have said there are other contaminants. They often produce low readings which is why any sane country does not have 0.00%.
Zero tolerance is not the same as zero blood alcohol. It means if you are over a level which cant be explained by these alternate methods you clearly have been knocking back the alcohol and there is justification for being charged. I dont think many of us have sympathy for those justifiably getting a criminal record but in India 151 innocents may have lost their job.