They are just very subjective, check into multiple systems that come up with adjustments and they don't really agree on the values. It is hard to say exactly how much more valuable a SS is than a 1B than a CF etc, then when you add P and C into the mix it gets really hard. Any discussion you see on the topic of positional adjustments almost always includes a disclaimer that the values are a bit suspect. Then any time you start comparing to the average or replacement player you get into even murkier waters, does Albert Pujols getting hurt make all hitters in the league better, because OPS+ says it does. I guess my point is not to argue if two pretty close in value players are better than each other, saying one guy is a 3.5 WAR player and another is a 3 WAR player really doesn't mean much because the margins of error are just too large. The other point is that no matter what stat you pick to judge players by it is going to be flawed in some way, so telling someone their stat is flawed isn't going to mean too much because your stat is flawed too. They all are flawed in one way or another.