Shell (top/bottom bonding) conflicting frequencies

I find myself in a bit of a predicament; I’m hoping someone can assist.  I am conducting a frequency analysis of an assembly that is comprised of shells and I am getting conflicting results depending on which side of the shell is flipped (bottom/top).  The shells are generated from a mid surface that is in the model so you would think that it would not matter which face is oriented towards the other face; however, this is not the case.  If I bond a green (top) to orange (bottom)it results in a lower natural frequency.  Inversely, if I bond a orange (bottom) to orange (bottom) it results in a higher natural frequency.  I am in SolidWorks Simulation 2011 by the way.  Through Simulation Help is suggest that the mid plane orientation color does not matter, but as I have described my results show that it does.  Can anyone help or explain to me what is occurring?

Chris