Simple. The 13w7 has a wide surround, but you also don't see the typical 0.5"+ ring on the outside which has all the screw holes.
Sd on the 13w7 is gonna be right around 650cm^2 (They spec it as 690cm^2, but that is using the 1/2 surround method), using the 1/3 surround method. Sd on the Ti 15, is 650cm^2, using the 1/3 surround method.
Trust me, they've got the same Sd. That was the whole idea of the 13w7. Get as much Sd as a wire surround, high excursion 15, and then have more linear travel, all in a smaller package. They succeeded in their goal.