Sorry for posting on a thread that's so old, but this is the destination for many a google search, I suspect. And you've asked a valid question that deserves an explanation.
There's a difference between rescanning for new disks at the adapter level and rescanning for update size information at the disk level.
Sending "- - -" to /sys/class/scsi_host/host[012]/scan is telling the SCSI host adapter to look for new disks on every channel "(-)", every target "(-)", and every lun "(-)". - this is the right thing to do if you add a new disk to the system while it's powered on.
Sending a "1" to /sys/class/block/sdc/device/rescan is telling the SCSI block device to refresh it's information about where it's ending boundary is (among other things) to give the kernel information about it's updated size. - this is the right thing to do if you change the size of an existing disk while it's powered on.
HTH