SSD vendors may change a product during its lifespan, not always for the better.

The SSD you’ve bought may not be exactly the SSD you think it is. That’s because of a common vendor practice of swapping out internal parts due to supply, pricing pressure, or other reasons.

Usually this practice has focused on the NAND flash storage modules on SSDs, and the vendor has met or exceeded the promised specification. 
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https://www.pcworld.com/article/3599339/why-you-may-not-be-getting-the-ssd-you-paid-for.html