A letter will go through several sorts its journey, in a kind of hierarchy of district, area, sector. These sorts are usually mechanical using the postcode. However, the letters and packets for the postman's walk will not be in strict order when he picks them up. He makes a final sort manually, using the full address, as the order in which he visits addresses may not agree with a sorted order of post codes.
So, the postcode is important for high level sorting, your postman made a mistake in his low level sort.
(I couldn't find a gender inclusive term for "postman")