Vanity sizing? Not many manufacturers these days actually produce clothes to the true sizes. M&S is one of the worst for this, I've found. If you look at official UK sizing measurements, then measure the actual garment - even allowing for the necessary cut and hang, there's a major discrepancy. I think they do it for financial reasons. It's crazy and silly but a lot of women are more likely to buy a garment if it makes them feel good about themselves by having a smaller size label on it.
BMI can be a pile of old tosh, anyway. One formula can't fit all of humanity. Don't worry about it!
It's all to do with muscle weight - a lot of good athletes are over on the BMI. Better to go off body fat pecentage cos you can be below normal BMI and skeletal looking and still carry an unhealthy amount of body fat around your internal organs.