BMI and Waist-to-Height ratios are widely recognized in the exercise physiology community to be fairly worthless for anyone other than the general population. They are just quick and fast guestimates that can be used when more accurate means (underwater weighing, bodpod, dexascan, calipers, etc) are either unavailable or when high measures of accuracy may not be necessary or possible (such as when you have to examine 100 people for a fitness test).
What the heck are you measuring with? 31 inch waist? I'm like a 29.5 and I would've thought yours would be smaller than mine!!