The variable A is a local variable in the function pushbutton2_Callback. Each time the callback executes a new A is created; that's why each A you see has a value only at last index.
handles.b should have what you want, except you have to store the handles structure back into the GUI after you modify the handles structure (i.e., after setting handles.a{index} and handles.b{index}), so that the value of handles.b (and anything else in handles) is up-to-date each time pushbutton2_Callback (or any other function) is called. You can do this by adding this line at the end of pushbutton2_Callback:
guidata(hObject,handles);
