It is amazing how effective it is to say "you're wrong" when it is coming from the "other" side, but when we say "you're wrong" we are deemed biased.
The difference I think lies in our interest to be correct. When someone tells us that we are wrong, we immediately go back and look at our data and try to present the data properly. When they are wrong, they stick to their guns and blaze through it. Conviction is often more important that truth.