celine - i agree with CarmenIbanez - if there was an actual bag on display with the pattern, it should have either been exactly what the pattern was or had something posted by it to note the changes the knitter made (including the handles, although that was a very visible difference). i would definitely call them if you can't get there soon, and speak with a manager if possible. perhaps the store doesn't even realize the displayed bag is different, at least they would be able to rectify that and let the knitter know to not make changes or make note of them on any future projects.
as for the other dilemma of avoiding this in the future, the measuring of the display item is a good idea, since the pattern should have finished sizes in it. i'm a very new knitter myself, but i try to read through the pattern and visualize it; for any measurements supplied (i.e., "cont in pattern til piece measures 9 inches") i usually check a tape measure to see just exactly how long that is. sometimes i measure existing items to see what those dimensions are for comparison, like the width or height of a purse i have or the length of sleeves. i think, too, that as time goes by and you knit more, you get a better grip on what the finished product will look like before and during the knitting.