It is an artifact of the electrical sensor - it just won't zero out like most gauges do. It wasn't uncommon for this to happen when brand new straight from the factory. You can try another sender - take an Ohmmeter with you to the part store and measure the resistance in the store before taking it home.

You don't HAVE TO run a mechanical line into the cab, there are several aftermarket electrical oil pressure gauges (gauge + sender) that will give your an accurate reading.

Mechanical gauges can be quite reliable - as long as you don't use that plastic pressure line that some gauges come with. Only use copper 1/8 inch pressure line - some parts stores sell it as a retrofit "kit" for the gauges that come with the plastic line. I've had that plastic line blow up before, spraying oil all over the engine compartment.