Excerpt from the Canon Boot Camp Manual
As you squeeze down a 21 megapixel picture 10 times to fit the 2.0 megapixel picture High Def TV (HDTV) requires, lines of information begin to overlap. This is extraneous information not needed to form the much smaller HDTV picture.
The HDTV frame size was so small that Canon could actually eliminate the two-thirds of information it was processing. This further reduced the demand on the circuit boards.
So Canon’s 5D (and 7D) circuits only output every third line and still retain a High Def TV (2.0 megapixel) picture.
5616 pixels wide / 3 = 1872 pixels wide
3744 pixels tall / 3 = 1248 pixels wide
Then, they worked a little magic and give or take a few pixels, Canon had a 1920 x 1280 picture.
RECAP: They reduced the volume of data coming off the sensor, making it possible for the circuitry to process HighDef, 30-frame-per-second video without putting cooling fans and faster processing cards into the Canon 5D. It’s called line skipping.
LINE SKIPPING: The 5D Mark II and the 7D are “line skipping” cameras when in movie mode. They skip two-thirds of the lines of information. The 7D’s sensor is physically smaller than the 5D’s but still holds 5184 x 3456 pixels.