I can look at a set of data, and decide A is better than B, but someone else can look at exactly the same data, but come to a completely different answer.
Why is this?
Because you’re a boogerhead. Nyah nyah nyah! 
Yes, yes I am.
Well that clears that up folks so I’ll be off. :rolleyes:
Because the data are interpreted differently.
If I say that France is geographically larger than Germany, you will assent, because we agree on the terms France, Germany, geographically, and larger.
If I say to you that, for instance, that Bush would make a better president than Gore, we may argue, because better is subjective. We can analyze better and come up with policy differences on abortion, defense, etc., but then those categories, though more narrowly defined, are still valued differently. No matter how small you make your inquiry, the desiderata are still debatable – it is, in this complex world, the data themselves that are questioned.
We can look at a given dataset and arrive at different answers to the question, “Is A better than B?” because you’re asking a subjective, qualitative question.
If you look at a database of, say, automobile specifications and ask people, “How many four-wheel drive vehicles are currently being produced?” you’re likely to get the same hard-and-fast answer from each respondent. On the other hand, if you ask, “What’s the best vehicle to drive?” you’re guaranteed to get a variety of responses.
The OP presumes “better” is a deterministic, objective issue, and there lies the confusion. “Better” is a value judgement. Only if we share the same values will we be likely to arrive at the same value judgement.
If we did, would Bush and Gore have had so much in common and so close a vote?
I think the whole country is “going centrist”.
Obviously, because you are wrong, and I am right. Or, your interpretation of the data is flawed and mine is accurate. Either way, the result is the same!
I have no idea what you’re talking about.