They all do that. Battery power is quite dependent on a lot of things... ambient temperature, humidity... the way the cam is being used... yadda, yadda. So you can bet they state battery power under the BEST conditions. As a result, what ever they say... take 40% right off they top and you will be a little more accurate.
What ticks me off is that my HC3 isn't even a year old and already the battery is showing signs of wear. Meanwhile, battery in my cell phone is 5 years old and still going strong