I think that's more that those shorts have a higher waistband so they're hiding the belly bit.

I think BMI can be useful if you know exactly what yours is and what your current fitness goals are-- it was very useful for me when I was extremely underweight for gauging how much weight I needed to put on and helped me see exactly how unhealthy my weight loss was (it was as a result of clinical depression, not an eating disorder!) but 90% of the time, I totally agree with you.