2) No, usually they would be computers that were bought new, but for some reason (valid or not) were returned to the seller, then the seller returns them to the manufacturer for credit who then checks them out and does what's necessary (hopefully) to restore them to new specs, etc.
3) Yes, you can get a great unit, but there's also the possibility that something was missed by the refurbisher (wasn't detected at the time) and didn't surface for you until after the usual shorter warranty has expired.