...digits in postcode (the only bit it ever checks,)
Not true. Apparently, there are degrees of authentication. I have two addresses, one for billing, one for delivery. Same postcode (ZIP, actually, as I'm in the US).
I occasionally get lazy and use the delivery address for the billing address, so I don't have to enter two addresses. I have had charges declined because the billing address didn't match (even though the card issuer has been informed that the delivery address is a valid secondary address for the account).
This doesn't happen all the time, seems related to the amount being charged, but not always. I'm led to believe that this may be an option, "require full billing address match", for the vendor, who gets a microscopically lower cost per transaction in return.