->
  var a
  ->
    a!
    a = ->

->
  var a
  ->
    a!

compiles to

(function(){
  var a;
  return function(){
    var a;
    a();
    return a = function(){};
  };
});
(function(){
  var a;
  return function(){
    return a();
  };
});

In the first function, because a is assigned after the call, a new var is created causing a to be undefined.

But in the second top-level function, a is not assigned, and so a is not undefined. (actually it is, but it wouldn't be if I'd assigned something before creating the nested function)

I wouldn't expect a new var to be created in the first one.