If the parts are out of specified ranges, any major customer could return them to the manufacturer. This is very costly for the manufacturer and the customer so it only happens rarely. If it happens much then Digikey will stop carrying the part.
For this reason I suspect they are almost always within spec, and if not then they are counterfeit, or they are damaged, or the measurement is not done it the way the manufacturer does it. Measuring milli-ohms is tricky, for example and any connections carrying the main current cannot be part of the voltage measurement, a 4 wire technique is required. Instruments must be accurate in the manner used.
There are cases where the conterfeit parts do get into the standard distribution channels, but this should be rare. It happens from time to time. But not often.