I think this revolves around the fact that it is impossible to accurately represent color of any sort through the web. Every monitor is different, as are user settings, as are digital cameras, as are the lighting conditions of the areas where the actual item is intended to be used. Blues tend to be particularly challenging for some reason. I have found it necessary 100% of the time to use Photoshop to darken blue items after photographing them, as they always come out far lighter in the image than they appear in actuality. This is probably due to the color temperature of the flash.
As a good practice, one should always sample the actual item when color is critical, and if possible, order more shortly thereafter to minimize the chances of slight variations in color that could occur between manufacturing runs.
RandyT