While you want the ideal frame size, I have bikes that are called 54 and bikes that are called 56 and bikes that are called 57(and some called medium and some called large and some 18"s and a 17"). They all fit me fine, with different stems and seat positions.

We tend to size for comfort, so I might put someone on a "close" bike to see how they feel on it. That does not mean I want them to buy that bike anymore than sizing someone on a bike above or below thier price range means I want them to buy that bike. There are only so many spaces for bikes in a shop. Most shops cannot afford every size and model in stock, much less on the floor.