Does a pressure cooker really cook food evenly?
As a couple of commenters already mentioned, the base of the pressure cooker gets hotter than the rest, and chicken is no homogeneous size.  

I would like to add that besides the size difference between pieces of chicken, there is also a difference in tenderness.  Breast meat is more tender so it will pressure cook faster, while legs and wings will be tougher and take longer.

There is actually a way that you can take advantage of this knowledge to have evenly cooked chicken - by stacking the chicken pieces.

Place the darker, tougher, chicken pieces at the bottom of the pressure cooker (closer to the hottest part) and lay breasts and any other tender meat on top of that.  Add just enough liquid to meet the cooker's minimum requirement (usually 1 - 2 cups).  The breast will steam on top (and cook more slowly) while the legs &amp; ect. will boil on the bottom (and cook faster).

Stacking chicken for even cooking is a technique that I published five years ago, as part of my pressure cooking lessons series.