Keep in mind wireless throughput in a sector is a "shared media"...
First, a quick explanation of the "shared media" aspect of the system. This knowledge base post summarizes the explanation that the SMs that are far away (or poorer signal levels) may be using more of the frame to complete their data transmissions, thereby degrading the overall sector capacity.
Second, there are some other customer experiences that you can read about here. These guys discuss some of the same points you've made here.