Every trade flow is recorded twice, by two customs administrations, using the same commodity codes. In a perfect world the two series would differ only by freight and insurance and by timing.
Sustained, one-directional divergence in specific codes between specific partners is therefore informative. It is not proof of anything about a particular shipment — valuation conventions, timing and re-export treatment all create legitimate gaps — which is why any honest use of the method publishes its assumptions alongside its numbers.