I had the same problem as you guys did first time seeing the movie about who was creating the dream worlds.
Ellen Page's character was hired to design all three levels and teach them to individual dreamers.
First Level: Rainy World
The dreamer was the chemist, which is why he stayed behind with the bodies when they went one step deeper.
Second Level: Hotel World
The dreamer was Arthur, again he stays behind to defend the bodies.
Third Level: Snow Level
The dreamer was Eames.
Fourth Level: Limbo
The dreamer was Cobb, which is why he knew how to get out and quickly get to Saito.
Ellen Page was present in each to act as a guide if they got stuck. Only she and the dreamer were to know the individual level layouts, which is why when she mentions the shortcut in the Snow Level, Mal is able to get to the target so quickly. Cobb knows the shortcut and thusly, Mal does as well.
