I've noticed the self-checkouts seem to have some safeties in place which the normal cashiered checkouts don't (because they assume the cashier is trained). For example, if I buy ten cans of the same soup, the cashier's system seems to let her scan one can ten times, whereas I can't do the same on the self-checkout, even the Walmart ones that don't care about the weight being placed. They have a "prevent double-scan" delay built in that is long enough I may as well grab the next can.