How so? They are incorrect, as subconscious inferences often are, but the conclusion is not a-priori unreasonable, and giving a group a handicap is often done because, all else being equal, they are not as good as the other group.
Again, you can (as I do) disagree with this assessment and think that it should be obviously wrong with a little bit of thought and knowledge of the actual situation, but as a subconsious inference it seams reasonable.
In fact, if you recall the "separate cannot be equal" ruling in relationship to racial discrimination, one of the arguments was that even if the separate accommodations were in fact equal (which they weren't) simply having black only schools creates a feeling of inferiority.
EDIT: Minor correction. After looking up the opinion for separate cannot be equal (Brown v Board of education [1]), it turns out that, in the opinion of the court, accommodations were in fact equal (or being equalized), which is what forced an examination of the separate but equal doctrine itself. I'll just leave this quote here:
"To separate them from others of similar age and qualifications solely because of their race generates a feeling of inferiority as to their status in the community that may affect their hearts and minds in a way unlikely ever to be undone." [1]
