GS1 barcode verification tests the correctness of the data in the barcode as well as the technical correctness of the barcode symbol.
The barcode symbol may be technically correct but there could be a problem with the data such as an unauthorised, incorrectly allocated or wrongly formatted GTIN.
Possibly there is a technical fault with the barcode that means it is not absolutely compliant with the specifications.
Depending on the nature and extent of the technical fault a barcode may still scan in some, many or even all situations. Not all scanners perform the same so some may forgive faults that will prevent others from scanning. A bar code with incorrect data in it may scan perfectly but it is conveying bad data that risks corrupting users' records.
A barcode that fails a GS1 verification test is suspect in terms of data or scanning capability and should be corrected before being put into the market.