is a possibility the gauge is flaky, they are not real accurite to begin with.
For giggles and grins, clean you cable ends ( both ends) especially the grounds at the rear of the engine ( near fire wall, right side, including the braided ground strap)
Loose grounds on the diesel trucks can really make things goofy with the electricals and the engine performance.
Also, check the firewall bulkhead connector ( 7MM bolt head ), make sure it is tight ( very common issue in this body) can cause some funky electrical issues as well
