I had trouble with this for a while. My trouble basically turned out to be different people using different defintions for linetypes. Some people were using ISO and others English linetypes. But you couldn't tell the difference because they both had the same name, shame on AutoCAD. We do both English and Metric drawings in my office so I renamed the linetype to have an e- or m- in front. On top of that I made sure everyone was using psltscale =1 setting for the drawings. After that it didn't matter what file it is referenced from, all lines had the same scale.