Car manufacturers follow SAE standards to measure the crank HP of an engine. Especially once SAE tightened the rules in 2005 or 2006, the numbers from manufacturers are much more consistent. It's much harder to pass an "underrated" or "overrated" engines through with the newer regulations. Also, all accessories that are standard equipment on the engine must now be on engine for the test (ex., no leaving out the power steering pump for the test to help boost HP figures).
That said, WHP is the one we care about. However, crank HP can be consistently measured in a very controlled environment (the engine is out of the car). WHP can have a lot of variables, as previously mentioned already in the thread.
Car and Driver article on the subject:
http://www.caranddriver.com/columns/...olution-column