How can replica shoes have working QR codes and serial numbers? Are they rejected factory pieces?

I’m trying to understand something about replica shoes.
Some sellers claim that even though the shoes are replicas, the QR code on the shoebox and the serial/style codes on the shoe (under the tongue) will still work and show up correctly when checked.
If that’s true, how does this work? Are these genuinely replicas with copied/duplicated codes, or could they actually be rejected factory pieces from the official brand that failed some quality-control/qualification test?
Would appreciate some clarification from people who have experience with this.