Simple answer is because the Gcon has a more compliant suspension (meaning it's not as stiff) as indicated primarily by it's larger Vas and secondarily by it's lower Fs. The Vas of the Gcon is almost twice that of that SA, since they are both 15's (which means Sd will be approximately equivalent) that means the compliance of the Gcon's suspension must be about twice that of the SA. And although the suspension of the Gcon is almost twice as compliant, the SA's moving mass is nearly 60% heavier, which is why their Fs' are only ~10% different. Vas plays a significant factor in determining how a driver will respond to a given enclosure volume, Qts and Fs play dominate rolls in determining the shape of that response. Generally speaking, a driver with a larger Vas is going to need a larger enclosure for a given response/alignment. If we place both drivers in the same enclosure, the Gcon is going to "see" that enclosure volume as being relatively "smaller" compared to the SA, which is why the response for the Gcon rolls off a little sooner (and it's group delay will be a little lower)