I did a zip code search for my city (76067 & 76068)
76067 returns over 100 listings on 3 pages. On page 2 and 3 there are several listings from other states.
76068 returns about 10 with 2 from another state.
What's happening is that Retrieve Callsigns by Zip / Postal Code is searching the entire field as a wildcard, not just starting from the beginning of the field. Because there are ZIP+4 entries in the database, it will return results if the ZIP you enter matches any part of the field, thus the occasional out of area result popping up.
In your 76067 example, we see two results from Raleigh, NC being returned. If we take a look at those two call signs, we find that '76067' is contained within the ZIP+4.