Definitely something is being recorded differently. The help page has a note about unique users. Maybe the Zip code map is not filtering unique users.
Note: This graph displays the number of unique users in each source location (i.e. postcode, city, and country). This means that a single user can request directions to a business 100 times in the aggregation period (7, 28, or 91 days), but we would only record one request.