Our MySQL is read heavy, but iostat reports that almost no reads are taking place. How come?

According to MySQL's STATUS command, we have about 500 reads and ~20 writes to our DB per second. But iostat is reporting that ~70 writes (w/s) and ~0.5 reads (r/s) are taking place on the corresponding device. Why isn't iostat showing all the activity that should be caused by the SELECTs? Does this mean that they are hitting some cache and that's why we are not seeing them? If this is the case, how can I tell? (The filesystem the DB is on is a BBU Raid 10 with SSD discs.)