Pascal strings store the string's length by its data, whereas fat pointers store the length by the address of the data.

The main difference is that if a string's length is by its data, you can't easily construct a pointer to part of that data without copying it into a new string, whereas if instead the length is by the data's address, you can cheaply construct pointers to any substring (by coming up with new length+address pairs) without having to construct entire new strings.