Yep, if it looks too good to be true it usually is (with the exception of David Silva of course).
Different sized memory chips in general (within certain bounds) are manufactured all on the same die. They test batches of them to determine the maximum safely usable capacity and then market them as such. In fact for marketing purposes they may even downgrade bigger chips just to fulfill sales targets.
The thieving sods that sell them on the likes of eBay and Amazon simply reprogram them to access the extra memory although they tend to be very unreliable.
They do similar with CPU chips too in terms of their speed rating.