For your general information, I would highly recommend working the attached document so you may know how a pre-loaded fastener really works in Creo Simulate. If you have not done this method properly, and confirmed the right pre-load, then there's no use questioning the results.
http://communities.ptc.com/docs/DOC-5290
Once the right pre-load has been confirmed, then run the model again with all external loads. This will be a total of three runs, two for the pre-load and one for the final.