Aj,
You have to rememeber that if you read 4ng of THC you are positive and 3.9ng you are not.
This is simply not the case there has to be a plus or minus factor in any testing device. The Chinese want this plus/minus factor quantified and together with SBN (DR Sun I suspect) they have devised a plan to define this variation. After they have and MPS is satisfied, then Chinese approval will be given.
That's my take. Hope it helps.