Did you know that the certifications awarded to albums do not reflect how many copies of those records were actually sold in stores?
Certifications generally reflect shipments from labels to retail outlets and not the purchases made by consumers from those retailers. In addition, these shipments may include sales made by record clubs, such as the now defunct BMG Music Club, where music is sold directly to consumers by label distributors.
For an album, DVD or single to be certified, a record label has to request and pay for the certification. The label has to provide all legal documentation that prove the shipments of the record to retailers as well as direct-to-consumer club sales, and these figures are then verified by the RIAA, for instance, in an audit.
It is impossible for a record to be over-certified. However, it could be under-certified if the respective label does not pursue the latest certification details. Hence, the record may have shipped more than the figure indicated by its certification and it is the responsibility of the label to ensure that the information is regularly updated.
So which organisation is responsible for record certifications in the US and Canada? The RIAA is currently in charge of that task. On the other hand, retail sales are monitored by Nielsen SoundScan.