No timezone being returned for some postal codes

http://where.yahooapis.com/geocode?country=USA&flags=JT&postal=84044&appid=[yourapikey]

We have quite a few postal codes that are not returning timezones. Below are a few of them

58460
38372
78154
84044

Is this an issue with the data or is there something else that I should be specifying on the call?