Barcode quality

I'm currently generating a UPC barcodes in SVG (to be printed at an exact size in mm) However when using a barcode verifier/quality scanner the barcodes always come out as a fail - the scanner grades the barcode quality by B to F grade.

Any idea why this could be? The library seems to gear towards outputting the barcode on screen by defining the barcode widths in pixels - maybe this is a problem when you are outputting small barcodes or outputting to a specific size in mm because it's not precise enough?

Checking another tool https://www.jacoballred.com/barcode/index.php?code=021200300608&encoding=UPC-A&fg=%23000000&bg=%23FFFFFF&multiplier=0.5 This is defining the widths as inches and seems to be using a much finer decimal place.

该提问来源于开源项目：lindell/JsBarcode