help! about results of Allen Taflove's 3D UPML matlab code

hello everyone. I used the Allen Taflove's 3D UPML matlab code (the link is http://www.artechhouse.com/Default.a...c/taflove.html)
but the simulation results confused me. Why the Ez look only has positive value and almost no negative value in the cartoon window. In my opinon, it should be alternation by positive and negative value of Ez, just like wave transfer.
can anybody explain it ? thank you very much.