Thanks for correction.
Yes, that may be one reason (together with speed and RAM limitations of various devices). But since this works transparently you never know. For example even if you choose bigger blocks then due to partition table and other alignment, beginning of such big block may not align with beginning of internal block so you'll get multiple writes and read-modify-write anyway. I guess there is probably some sophisticated controller on the card to handle wear leveling and bad blocks and there is probably also some spare area with unused blocks so I guess it is same like with harddisks now - you don't know the real geometry, it is just array of blocks and any guesses may be wrong or may even change after card is used for some time.