From Wiki:
The BMI is meant to broadly categorize populations for purely statistical purposes. As noted, its accuracy in relation to actual levels of body fat is easily distorted by such factors as fitness level, muscle mass, bone structure, gender, and ethnicity. People who are mesomorphic tend to have higher BMI numbers than people who are endomorphic, because they have greater bone mass and greater muscle mass, respectively, than do endomorphic individuals.
Similarly, ectomorphic individuals could conceivably receive a reading below the normal range, when in fact their body type makes it healthy for them to be thin. In fact, ectomorphs could obtain healthy readings even when their body fat percentage is higher than is healthy, as their low lean mass will lower the BMI.
From Archy:
I am a mesomorph. Even when I am in good shape and lifting hard, I can only get down around 22, 23; but my body fat was down to about 12% at that time. So the BMI #s do not necessarily correlate to actual body fat. For your h/w and activity levels, you are probably a bit less than normal women in the Body fat dept. And, you look like a mesomorph, is that how you would catagorize/characterize yourself?