On the surface, one cannot tell that the DRAMs are substandard. Even though the DRAMs may work, and may very well have been sold as new, they often will not meet the Grade-A standards for speed, performance, durability, longevity, or data retention. Cheap PCBs DRAM quality is very important, but so is the quality of the PCBs they're mounted on. If a manufacturer is attempting to maximize profits by using cheap chips, one might suspect that cheap PCBs are being used as well. Cheap PCBs cannot maintain maximum data throughput and overall reliability is a real issue.
Testing is expensive and time-consuming.
Some memory manufacturers would simply rather deal with a manageable number of product returns than incur the cost of testing each module, carefully and individually. These memory companies either don't consider or don't care about the costs associated with the end users' wasted time dealing with defective modules. Substandard Manufacturing Believe it or not, there are still some computer memory upgrade products being assembled by hand.