Don't know, but probably the maemo staff is spread so thin that N950 is very low priority. So it might very well be PR1.3 is not coming. In fact, I think it's likely it won't come to N950, as we never got even PR1.2 final.

However, it is apparently possible to upgrade N950 to PR1.3 by changing the repositories to N9 repos and going from there. I won't give instructions here as I haven't done it myself. I just read it in #harmattan so at least it seems possible.