I can think of at least two reasons:
1. The bike is not always outfitted at it appears on the website. For example, the tires it comes with may be different than what is pictured. That could affect weight.
2. Bikes comes in different frame sizes. Also, larger sizes may have things like wider bars than smaller sizes. Different sizes of frames and other parts=differing weights.
Based on my years of experience on this forum, I can totally see someone getting uber pissed that the bike they bought on line was 4 oz. heavier than advertised on the web site.