Please see the code and comments below for explanation...

   // Consider the following default settings
   $.ajaxSetup({
    data:{'var1':'A', 'var2':'B'}
   });
   
   $(document.body).click(function(){
    // TEST 1
    $.get('/test1/', function(r){ });
    //RESULT: GET http://localhost/test1/
    //WRONG: Missing var1=A&var2=B
    
    // TEST 2
    $.get('/test2/', null, function(r){ });
    //RESULT: GET http://localhost/test2/
    //WRONG: Missing var1=A&var2=B
    
    // TEST 3
    $.get('/test3/', {}, function(r){ });
    //RESULT: GET http://localhost/test3/?
    //WRONG: Missing var1=A&var2=B
    
    // TEST 4
    $.get('/test4/', {'var3':'C'});
    //RESULT: GET http://localhost/test4/?var3=C
    //WRONG: Missing var1=A&var2=B
    
    // TEST 5
    $.get('/test5/');
    //RESULT: GET http://localhost/test5/
    //WRONG: Missing var1=A&var2=B
   });