That does make sense. Fair Oaks is Postal ZIP Code 95628, Carmichael is 95608 or 95609, and North Highlands is 95660, none of which are 958xx.
Obviously, postal codes do not cities make, but going back to the ZIP code system, when it was set up in 1963, it was set up like this:
First Digit = region of the United States.
Second Digit = State, US Territory, or a region within a larger state. (CA was large enough to have regions for everything from 900 to 961)
Third Digit = City (if large enough), or defined smaller communities and rural areas considered to be in the vicinity of a large city.
So, Sacramento was assigned 958xx (other than the California State Government Offices which were assigned 942xx), and smaller communities or rural areas around Sacramento were assigned 956xx or 957xx.
Unfortunately, Sacramento never formally annexed everything 958xx.
This is in stark contrast to San Francisco, where everything is 941xx, Oakland, where everything is 946xx, or San Jose, where everything is 951xx, except two small communities annexed after 1963, when the ZIP code system was set up.
https://en.wikipedia.org/wiki/List_of_ZIP_code_prefixes