Memory cards are not identical, delicate pieces. Quite the opposite: production output varies a lot, and the process is all about efficiency and recovery.

How cards are sorted and labeled

Product labeling is based on post-production testing, which sorts the cards into bins.
The raw product is a commodity, and the quality and quantity available on the market vary.
One commentator described a particular memory card vendor's output as "catch of the day".

What this means for performance

The performance of a particular microSD card can differ from a card by another manufacturer, and even from the "same" card made by the same manufacturer.