What makes you think that?

Look here: http://en.wikipedia.org/wiki/Address_Verification_System

I had remembered from an earlier implementation that it would check everything and that we could approve transactions where numerics match but text part isn't the same; but possibly it was simply choosing what to do if street address was ok (for the number) but the postal code was wrong/missing, as here most people don't care about their postal code and may enter it wrongly.