We usually supply barcode numbers in 13 digit EAN-13 format, with a leading 0 (eg 0799439112766 ). However, these numbers can also be used in 12 digit UPC-A format without the leading 0 (e.g. 799439112766 ). The actual bars produced for both of these are identical (shown in the picture below).
Outside of North America it is common to print the barcode on products in the EAN-13 format. However, some retailers have software systems that prefer numbers without a leading 0, and therefore they will prefer to use your barcode in the 12 digit format (e.g. 799439112766 ). This is fine – the number is still the same, still unique, and will work OK. You can let the retailer decide which version of your barcode number they will use.