Can't understand if deallocation happens?

[Suliman via Digitalmars-d-learn]

  	string str = "abc";
  	writeln(str.ptr);
  	str = "def";
  	writeln("last data: ", *(str.ptr));
  	writeln("old data: ", *(str.ptr-1)); // print nothing
  	writeln("old data: ", *(str.ptr-2)); // print c

It's look like that there is some gap between data, because `d` 
minus 1 should be `c`, but `c` I am getting only if I am doing 
`-2`. Why?