Well, keep in mind non-fat mass is different from lean mass. The part of you that isn't made up of fat ALSO includes things like bones, muscle, and even water.

For every bit of stored fat your body contains, it also hangs on to some water with it. So it is to be expected, unless you begin to work out, that your non-fat mass will decrease as you lose more weight.

This is why it isn't realistic for someone who is still pretty far from their goal to crunch final numbers like that.