This is maximum power draw, not typical usage. I believe either Home Theater or Sound and Vision did a real world test of LCD vs. plasma for power consumption and their tests showed that the difference between the two technologies was neglible.
It is exactly as stated... A LCD draws 100% of lamp power 100% of the time. In comparison a white scene with a plasma draws 100% power, but a black scene draws very little current. The average is somewhere between the two, with a lean towards less power since most video is darker than it is lighter.
The results showed that LCD did still maintain an advantage, but it was nowhere near what had originally been believed to be.
Interestingly, because LCDs must charge a pixel to make it black, the darker an image the more power a LCD display will use.