I think there are a few reasons why they lie, and a booker/client/marketing person probably has more developed reasoning than I have had the opportunity to make but, practically speaking, if you go to a casting with a comp with 5'6.5" on it, that shit is getting thrown out. I mean, there's already a pile of 200 cards (at least) and narrowing it down is quite a job so any bad number like that makes it an easy cut. Also, I think bookers only submit girls to client inquiries(?) that fit their requirements (re: brunette, cool, fresh-face, 5'9"-5'10"), but I am kind of guessing because I am on the other side of that particular curtain. Why VS lies? To be consistent with the other lies and to maintain the esoteric properties of le mannequin which is and always has been tall and thin. And your average person isn't going to notice she's 1-1.5" short. I'd say as far as all working (top agency, major market) body models are concerned, the height break down is something like:
5% 5'7"-5'8.5"
15% 5'9"
40% 5'10"
35% 5'11
5% >6