I haven't done a sample problem to check it, but my guess is that the small feature you added forced cosmos to refine the mesh at that location. That will give you a different result than a coarse mesh, especially for stress. Not as much for displacement. I don't believe you can specify mesh size in "express", so you may be out of luck in proving to yourself what the stress really is. SolidWorks intends "express" to point engineers towards problem areas, not to substitute for the real CO$MO$/Work$ package :)
Question for users of Cosmos/Express 2004/SP1.0
>
> Why would adding 0.03 fillets to the circular edges of four mounting holes
> both sides (i.e. restraints) reduce the FOS by a factor of two? I was
> expecting the FOS to remain unchanged or to improve slightly. High stress
> areas occur at two of the four mounting holes 2x2 pattern, cantilevered > load.
>
> Kman