Does a pressure cooker really cook food evenly? My mother uses a 5 liter pressure cooker to cook chicken pieces.  

She heats it for 10 minutes, switches off the stove and waits for the steam pressure to go down before opening the lid.  

Even though this is done, some chicken pieces, although seemingly cooked; when you chew it, it feels like chewing gum and does not 'disintegrate' well enough to be swallowed. Same way you don't feel like swallowing a chewing gum because it does not 'disintegrate' into tinier pieces, for example, the way cooked rice 'disintegrates' into a mushy semi-solid before we swallow it.  

There are other pieces of the chicken which do get fully cooked, and they are not like chewing gum. They 'disintegrate' and can be swallowed comfortably.  

I asked her to cook it for longer. She cooks it for 15 min now-a-days, but the result is the same. Some pieces end up like chewing gum.  

She believes that since it is cooked in a pressure cooker, the heat and pressure should be even, and therefore all pieces should get cooked evenly. So even though I proved to her that it isn't evenly happening, she still remains stuck with the belief that it should cook evenly, and she's doing nothing to solve the problem.  

So:
1. Is the food (especially meat) cooked in a pressure cooker really supposed to get cooked evenly?
2. Is boiling the meat in a pot (instead of pressure cooking it) the only other alternative?  

UPDATE: From the answers and comments. Yes, Mom says that first we wait for the steam to come out of the pressure cooker's vent, then we put the weight over the vent and that's when we start counting the time taken for cooking. We can't really depend on the number of whistles, coz it varies among pressure cookers.
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.