I don't remember if they measure the weight with a full or empty tank (I should've asked one of the inspectors the last 2 years). If it's a full tank, it's the teams' placement of the additional weights to bring the car up to 3400 pounds (plus the penalty for a light driver) that would make the car break loose. If it's empty, then the cars are about 33 pounds lighter (give or take a pound) over the rear axle.
In any case, there is, if I recall, some on-track adjustability of the weights. I don't know how much time the Busch teams had to adjust before impound though.
One more thing to consider; it's a LOT warmer in Vegas now than either the average or when they tested. That heat makes for a greasy track.
I know they SHOULD figure the difference, but did they is the magic question. There's a big difference in set-up between this track and the super speedways, so that data is basically useless.
You're right about the different temps. LVM commented up thread that it was snowing last year. ;)