don't TG guys use to turn off electronics aids (except launch control)? in these conditions a 4WD car (GT-R) is still as fast as it would be with aids on, while a car like the 599 is not exactly "controllable" without them so it's really hard to achieve the best in few laps; also, they did a standing start... which is your worst nigtmare if you're driving a front engined RWD car with almost 700hp, while -i'm not sure about it- in fiorano they did as many laps they wanted with all aids on, without the standing start.
moreover, TG has recently changed the stig so we don't know "which" is faster
then, i bet if they give the GTO back in stig's hands he would acheive a very different lap, faster or slower...
i believe that all these factors together really can justify that differene