It mostly comes form here. Long story bit shorter (but still quite long): Back in the early days of Windows Phone 7, some devices had SD cards slot. It wasn't mean to be user accessible, it was meant only for manufacturers, to offer bit of flexibility for different markets. The phone at first boot detected, locked and encrypted the card. Since then the card wasn't just another kind of storage - it became a physical extension of system memory, indistinguishable and irremovable (removal of the card corrupted phone memory and required full reset). Such usage (as system drive) put unusual requirements on performance of the card. Any delays in random-access resulted in unacceptable lag for the entire phone. So, Microsoft tested many cards and found out that top class-rated cards don't perform well. The list of cards that did deliver best random-access performance unfortunately consisted of models not available in retail.
My statement in comment was clearly an oversimplification because I remembered it wrong. I should have been more clear:
SD card class rating measures only sequential performance (eg films). Higher class-rated card in random-access performance (eg apps) can be beaten by lower class-rated (or unrated) card, but it's not an absolute rule.
Only comparing cards of same era (and price range) makes sense. By no means I wanted to say that cheapest class-4 noname will beat most expensive class-10 in anything (although I did).