Rregarding an example in "function invocation and the execution stack"

Rregarding an example in "function invocation and the execution stack" .

function a() {
  b();
  var c;
}

function b() {
  var d;
}
a();
var d;

why isn't "var d" stored in the memory of the global execution context? I thought functions and variables that are not inside other functions were supposed to be attached to the global object in the global execution stack.