Ski size is dependent on hight and weight as well as ability. If the sales person asks the right questions and they are answered accurately they should be able to get you in the correct ski. Without a doubt people do misrepresent their skiing ability but who is to blame for that? I have also had customers that just purchased a ski because they liked the color regardless of what is recommended.
I think a pair of friends of ours wins this, he was put in 120 flex salomons, 2 sizes to big (29.5) he skied 25.5 for race, he found them to big when skiiing, so he went to another place, they put him in 120 XTD atomics, but I came into store as they were doing the final stuff saying he probably should be in a 130 at least, I know he has a ski racing background. he has never been a gear freak, and would just ski whatever was given to him
long story short, he got the 130 atomics, and we went skiiing, I havent skied with him before.. he is hands down the best skier I've ever skied with, both on, and offpiste, he flexes the daylight out of them. When talking about it later, it turns out he had about 100+ days a year racing/freeriding, complete long days, bell to bell, for almost . 20 years growing before his now 4-5 year hiatus(did I write that correct)
way to modest, way way. only person I've seen hipscrapeing out of the lift on skis he never skied and borrowed, on touring boots, after a few years off... it was a good trip, and such an awesome couple to ski with. when asked about his skill he would say not very good at all, and never caring about gear.. yeah you get weird results, first store didnt even bother to measure his feet, just pulled a pair from shelf he said