If this is a Workhorse chassis with a GM engine, it considers several factors in predicting the need for an oil change. Mileage is part of it, but it also factors in operating conditions such as RPMs, engine temperature, etc. to evaluate the probable condition of the oil.
The following quote is from a research paper on oil change prediction methods:
The General Motors (GM) oil-life system, first introduced in 1988, determines when to change the engine oil and filter based on the operating conditions of the engine. The technology does not actually monitor any single quality or physical property of the oil. Instead, the oil-life system records engine revolutions, calculates engine oil temperature and engine oil contamination content. The rate of degradation of the engine oil is calculated by assessing the severity of engine operation as a function of the calculated engine oil temperature and oil.
If yours is calling for a change at 1000 miles, I can only guess that the engine is run only briefly and seldom gets fully warmed up, and maybe often operates at idle as well. Both of those lead to early life contamination of the oil.
I have a GM engine in a big SUV and oil change intervals typically run to 12-15 months and 11,000+ miles, but it runs at highway speeds almost exclusively.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL