If you notice your scanning device is scanning 13 digit barcodes as 12 digits you may need to enable the 'Check digit' setting on your device
Open the 'Scan tools' application
Tap the 'Barcode' tab
Tap the arrow next to UPC/EAN
The UPC-A, UPC-E and EAN-13 options should all be set to 'On'
If any are set to off, just tap the option and enable it