Vanity sizing

I sneaked a peek at my measurements at the bridal store out of curiosity. The sales lady had subtracted an inch or more from every measurement. (I remeasured myself at home to make sure: yep, at least an inch had been knocked off of my bust, waist, and hips.) Why would you do that. Is this some tailoring practice I'm unaware of? Below is the size chart for my dress:

But this chart is completely inaccurate. The waist of their size 6 is actually about 28 inches, not 25.5. I guess it's like how guys' size 32 waist pants are often more like 33 or 34 inches. I can understand making a size 6 larger to accommodate Americans' expanding waistlines, but shouldn't the measurements listed in the size chart be adjusted to reflect this?

Here is an interesting excerpt from a term paper on vanity sizing. Essentially women's clothing used to be done by age and start at size 14, as in fit for a 14-year-old. Yes, the smallest size was a 14. The bust of a size 14 was 32 inches. Now a size 00 has a bust of 32 inches. This timeline pinpoints when sizes smaller than a 14 were introduced to cope with vanity sizing: