fstrim vs. mount -o discard: How come discard.... doesn't?

I've noticed from time-to-time that running fstrim on mounted ext4 filesystems yields massive amounts of trimmed space even when these filesystems are mounted with the discard option. I thought the discard mount option was supposed to trim ext4 right away. Why does this not seem to be the case? NOTE: scd1 is on a pass-through disk behind an Areca RAID controller if that makes the difference.

# fstrim -v /mnt/cache/
/mnt/cache/: 1351144587264 bytes were trimmed

# mount | grep cache
/dev/sdc1 on /mnt/cache type ext4 (rw,noatime,nodiratime,discard)