Because it's all about balancing stats, so as your gear changes certain items that used to be big upgrades may no longer be. I believe maxdps & wowhead just use preset weighting factors for stats, so they don't know whether you are hit capped or expertise capped, hence why Fool's Trial (which has hit rating) may show up higher on there than from other sites. Also, some other stats just become less important as your gear changes because that gear will have more of it already. And some, like armor pen, become worth more as you get more.
Out of the places you listed, I would believe Rawr since I believe it takes into account your current stats, whereas the others may not. I haven't checked maxdps in awhile, so I could be mistaken. The most accurate gear tools I've found are the spreadsheet from EJ and the warrior Simulationcraft tool from there as well.