For one thing, there's the issue of the actual rating process. Before 1971, engines were factory rated using a process defined by the Society of Automotive Engineers as 'Gross' horsepower. This figure was calculated on a test stand with no intake, exhaust or power-robbing accessories attached. After 1971, power levels dropped as manufacturers re-rated engines using the SAE's 'Net' process, which added intake and exhaust restrictions and the load of engine accessories, like the alternator and power-steering pump.
But there must be an equalizer, right? How do we know how much horsepower our favorite classic muscle cars really put down as compared to our latest batch of favorites from today's showrooms? We suggest you check out this informative article from Hagerty, which explains why a horse is a horse, of course, and why we shouldn't necessarily take off our rose-colored glasses even when presented with clear-as-day performance numbers that show things are better today than they had ever been in the past.