STL files are basically unit less. When you import it, the file adopts the units you have setup in your defaults, or your start file. Make a 1" cube stl it out then bring it into a part set in mm and a part set in inches. When you measure one of the straight edges you should see what is going on.
Model a 1" cube ==> export a 1 unit stl file
Model a 25.4 mm cube ==> export a 25.4 unit stl file
Import a 1 unit stl file ==> creates a 1" cube or 1mm cube upon import depending on your settings
Imprort a 25.4 unit stl file ==> creates a 25.4" or 25.4mm cube upon import depending on your settings.