My opinion; Inexperience, coupled with a fear of flooring the accelerator.
I've seen some pretty poor 60' times at the track from the new GTs, but never seen a crappier ET or trap speed. Maybe there was a kitten jammed under the gas pedal. :)
They are running fairly consistently in the mid-high 13s at New England, with the only real crappy times I've seen coming from a poorly launched automatic.