Originally Posted by
Yvaelle
This is only true if you are looking at a broader range of rating, but by looking at 2600+ you are filtering out all the random comps - and selecting for only a handful of 'best comps'. So Spriest/Fmage/Tree is pretty much the only Shadow comp from 2200->2900 (Spriest/Rogue/X isn't a thing in WoD), but while Warriors work with everything, above 2600 it's almost entirely Arms/Frost DK/MW. So from 2200-2600, you can slap an Arms warrior on any pairing and make it work (not true for Shadow, by contrast), but above 2600 as shown here - even Arms warriors only really have one comp.
So, your argument about more viable comps = more representation would be true if this were a 0->2600 analysis, but above 2600 (and probably more like above 2400 or 2500 or something) that argument will cease to explain the overwhelming Arms representation (the real answer is that they were over-buffed for PvE and it affected PvP).