Usually they get my size right... except when it comes to jackets and bras. I have sort of large boobs (32D), a small waist (26-27), long legs, and fridge shoulders (36). When I got my burberry trench, the SA brought me a 0 (way too small once I'd buttoned it up), then a 2 (still too small), and so on. She was a little reluctant to go past the 2, but once I buttoned up the larger sizes she was happy with how they fit my midsection - she was worried that it'd be too big. I think I settled on either a six or an eight.
For about... six years I was convinced I was the same bra size. I relied way too heavily on finding my size with a measuring tape and as such, believed I could NEVER EVER be a different size. My good ol' 34B fit me from the time I was 9 until I was 15. And even then I kept wearing it until my boyfriend pointed out that I was strangling my poor chest. Every time I'd go to victoria's secret I'd have the SA take my size and time and time again it would be a 34B. One day, I decided to try a C. I was terrified and i don't know why. I felt like everyone was staring at me and talking about me behind my back. 32C was too small in the cup and the 34C fit my ribcage - barely. But I wore that happily until my stupid bras started stretching.
A couple months after that, I'm at bloomingdale's with my mom, finishing up Christmas shopping. Since my bras are getting droopy, floppy, and ill-fitting I ask if we can look through that section. I picked a few pairs of 34Cs, tried them on, and gave up. I HATED looking for bras. My favorite styles at VS had been discontinued, I felt strange about looking at any size other than 34B, and I just felt defeated. I'd more or less resigned myself to wearing ill-fitting bras until this SA brought different cuts and brands. They still didn't fit, so she thought for a second.
"You may be a 32D."
After the initial shock, I tried it. What a difference wearing the right size bra makes
So I guess it can sometimes be wanting to flatter the customer, or in both of my cases, wanting to make sure the customer wears the proper size and feels comfortable in it