Weight—it was measured differently back then. I can't remember what the old measurement was, but the new one is based upon a full tank, a load of average-weighing passengers, and bonus weight in the trunk to compensate for baggage.
Horsepower—this too is measured slightly differently. I can't remember how, but even today the standards are only estimates. Take your car to 2 different dynos and you'll get 2 different numbers. Also, remember that elevation may yield different results. I don't know the modern elevation for dyno runs that car companies use as a standard, but I think it's different than back then. I read this somewhere, but now I can't remember where.
Efficiency—not all the power got to the wheels back then. Even today, a significant amount of power is lost in translation. With new technology, more of the power gets to the floor.
Tires—stock tires are just better than they used to be. With decades of additional research, it's downright unfair to compare a '67 Camaro with a '02 model with stock rims and tires because new ones are comprised of new technology that make them handle the power better.
Aerodynamics—while this only makes a big impact at really high speeds, it still impacts quarter mile runs and other types of racing. On the dyno, this doesn't explain anything, but it is clear that newer cars tend to be built with the wind tunnel in mind. Isn't that why they've been redesigning the Volt?
Tuning—computers on cars have vastly changed the way our engines run. With better tuning ratios and numbers, we get better results. Using the same engine, we can clearly show that the tune makes a difference. All of you with modified cars know that chips and reprogramming tools, when properly used, are great ways to gain horsepower that was already built into your ride.