I think you don't understand statistics so well.
Take your meat-packing plant, and run a batch of horsemeat through it. Do a pretty good job cleaning, but you might miss a morsel or two. Run a batch of beef through it. The first bit through might have up to 1% horsemeat contamination (assuming things are kosher, as it were), but the last bits of cow might not have detectable levels of horse. Clean and run a batch of horse through again & you'll see cow contamination in the first bits.
One assumes the regulatory agency's sampling methods are reasonably random and that they'd look askance if a big batch of beef had similar levels of contamination throughout.
Think for yourself, schmuck!
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