PHC1 -- also keep in mind that there are a lot of games with dyno numbers. When I say "true horsepower," that's being dyno'ed with the side pipes bolted on, water pump pumping, no goofy adjustments to the machine, etc... but dyno games are for another thread.
It's important for you to understand that though when you start comparing/contrasting builds for your car. And it's not like the engine builders are trying to "pull one over on you" either -- imagine if you were building engines and your competitor's numbers were with open pipes and yours weren't. You'd look like a lousy builder unless you got a chance to 'splain.
Someone on here had a pretty funny thread a couple of years back when he insisted that his new FE be dyno'ed with his sidepipes bolted on. The builder really didn't want to do that, but he eventually got his way.