The variables involved in lap times are enormous. From driver, to weather, to suspension setups, the variances are enormous. I can't explain the discrepancy because I do not have all of the facts. The weight gained in a 911 is fairly minimal and may actually work to offset the inherent oversteer problems people are contantly fighting in that platform.

A single lap time comparison is not comprehensive evidence. Especially when discussing street cars on street tires driven by TV car commentators.

Edit: I did a little research and it turns out that the GT2 is lighter, has more power and comes with a more aggressive suspension package than the 911 Turbo AWD (X50 package). One is meant to be primarily street driven, the other is oriented toward performance driving or track day duty. The weight difference is over 300lbs(3500 for the X50, ~3100 for the GT2), the horsepower difference is 33 at the flywheel and the suspension is supposed to noticeably stiffer. There's your 1 second and seeing as how the GT2 has an inherent advantage in all performance categories save gross grip, I'm not at all surprised.