The GPU market is often rife with counterfeit cards. These products are often sold as having more memory than they actually do, or as having newer NVIDIA chips that are in fact reflashed older ones - or both. TechPowerUp has upgraded its diagnostics app GPU-Z in order to detect these techniques and report affected cards as fakes.
https://www.notebookcheck.net/GPU-Z-updated-with-the-ability-to-detect-faked-NVIDIA-cards.338240.0.html