There have been many discussions on this board about the 10-12x guideline - you can find about 3 of them in the "Hot Links!" thread atop the "Newbies' Questions" forum.

The reader's digest version is this: 10x your body weight is roughly the amount of calories your body requires on a daily basis to perform those duties and actions that it must to keep you alive (or your BMR - basal metabolic rate). This is the energy it takes to break down protein, to build enzymes and hormones, to regulate body temperature. To keep you alive. It doesn't take into account your lifestyle (active, sendentary, bedridden) or the exercise you do every day (walking, weight lifting, bicycling etc).

Eating less than your BMR puts stress on your body - if it cannot receive the fuel it needs to function it is also not going to receive the fuel it needs to burn fat.

The guideline comes from basic human biology - not from any one LCing (or other diet) book.

HTH
Nat