A bike being the "right size" has little to do with whether it FITS you well or not.
Any LBS person who sends you off with a new bike just because it's "your size" isn't doing their job. You could take 5 bikes that are all your "correct size" they will all fit you differently. You will have a different body position on each one. ONe might be a perfect fit and another might leave you in pain after a couple hours of riding. Yet you are told they are all "the right size". The right size often is just a beginning point in the search for a good fitting bike- it only refers to ONE measurement on a frame- the seat tube. It doesn't have much to do with angles or arm reach or handlebar width/height, and a dozen other factors. Happily, some of these factors can be changed on a bike, but others you can't do much about.
This is an interesting article on frame sizing (with some definite opinions):
http://www.sheldonbrown.com/frame-sizing.html
It also has some good links on the bottom to other articles by other people.