The definition of "shortest" can be controlled via the TraceConfiguration.ShortestPathNetworkAttribute property. Typically, Shape Length is used. One counterexample might be if you are looking for the shortest path through an underground system. You might prefer a longer path via duct banks, versus a shorter path through a trench that requires excavation, since the excavation might be more costly. In this case, you would set up a network attribute that represents these costs.

Reference

ShortestPathTracer Members
ArcGIS.Core.Data.UtilityNetwork.Trace Namespace