I don't know a lot of details about these battery life tests that are reported, but I think that real world usage is always going to result in faster battery drain. For example, when they talk about 8 hours of HD video playback, that's all they're doing--putting a video on (usually streaming via wifi), and just leaving it alone. They're not surfing, accessing other apps, doing other various tasks with the device. All of these other activities will tend to use more battery, because doing additional things requires CPU power.
I have always found that with real world usage, I get about 5-6 hours of total screen-on time (which is probably the best way to measure how much I've actually been using the device). I wish that tech reviews would include more real world tests, but then again, real world activity can vary from person to person.