The human subjective experience (which is what the rationality behind an individual vote comes down to) should not be distilled down to statistics of probability. It would be extremely egregious and disrespectful to do so, and it carries the implication (unintentional or not) that minority groups are robots of specific kinds of programming that trends in certain directions. Someone voting against their own interests ALREADY REPRESENTS THEIR OWN INTERESTS BY DEFINITION. If that person's interests reflect the majority interest, then they represent the majority of their demographic, even if they don't vote the same way the majority votes.
This has nothing to do with the ability to make an objective prediction that is based on facts. This has to do with tact, and how you handle conversations surrounding people groups. If I'm a Hispanic, and I told you that I voted for Trump, without providing any additional information as to why I voted for him, and then you told me that I likely don't represent the majority of my race because most Hispanics aren't likely to vote for Trump, I would seriously question your racial sensitivity.
To be fair, this is not just a problem on poliGAF, but with pundits and the media and general. I have never liked how they've discussed voting trends when it comes to minorities, but this was just an opportunity to talk about it.