Even if there's some delay, almost all self checkouts are roughly trying to square what you just scanned, with the weight of the stuff in the "already scanned"

Even if there's some delay, almost all self checkouts are roughly trying to square what you just scanned, with the weight of the stuff in the "already scanned" area, and optionally throwing errors after some threshold (the worst offenders turning off the scanner altogether until the last scanned item is weighed/validated).