Not sure why that should be a surprise, you can shave a full second off a 2012 GT500s time just by switching to slicks and skinnies.
The fact is that not a single magazine or owner, to date, has been able to dip into the 11s stock, or equal GMs reported times.
I believe the two tuners that have tested the car reported 12.3 and 12.5. Magazines have been anywhere from 12.1 to 12.9, and that 12.1 had the significant advantage of being tested at 4400' elevation and then correcting the times for the air density (i.e. it didn't actually run a 12.1, it ran a 12.6 and they corrected it down to 12.1).
Best owners run has been 12.02 IIRC, and many are in the 12.5 neighborhood (we'll ignore the guys that have run 13s for being poor drivers, although that's still a strike against the Camaro since the GM fanboys have been saying for months that the Camaros launch control/PTM, wider tires, etc would allow even your grandmother to run bottom 12s all day long).
Laguna/VIR/Willow/etc head to head tests are going to start pouring in any day now. If GM has the decided road course advantage it shouldn't be a surprise, in that case you've got a 55k dollar GT500 that dominates at the strip, and a 56,300 dollar ZL1 that has at least some advantage at the roadcourse, and you pick your poison (although it's pretty hilarious how many of the bowtie faithful have all of a sudden decided that handling is all they care about, despite the fact that .01% of them have probably ever turned a tire on a twisty track).
It's also pretty funny how many people think ANY 4300 pound raceweight vehicle with a heat soaking positive displacement blower is a "roadcourse car". GM makes a car that is GREAT for roadcourse work, hint: it isn't a Camaro.
So it's likely to end up Shelby:1, ZL1:1. But if the ZL1 gets beat in the hands of an independent tester at VIR or LS? If I were GM I'd be sweating.