It's a complete bullshit number that they pulled from their asses.  Many of the VS Angels will end up a couple of pounds underweight for the big VS Shows that end up on TV.  But they are that weight for like a day or two and as soon as the show is over they will quickly put 10 pounds or more pounds back on because they are basically boxers cutting for the weight in.  They have dehydrated themselves because that helps define their muscles.  A random Tuesday in August though they have a 19 or even 20 BMI.