58% of real UPC-E barcodes also pass the EAN-8 check, so our scanner tests both sums

A UPC-E barcode carries the check digit of its expanded UPC-A, not of the eight digits the scanner hands you. Over the whole 2,000,000-code space, 58 per cent satisfy the EAN-8 sum anyway, and 42 per cent get thrown out by a validator that only knows one family.