Rawr (and the various spreadsheets that perform similar functions) looks at what effect it will have in conjunction with your existing gear.
BiS lists are generally made in one of two ways :
A)Look only at the items in that slot, which essentially means "If you are otherwise naked, Item X will do more than Item Y"
B)Assuming that all other slots are BiS. In that case it generally ignores things like whether or not you are hit or haste capped from the rest of your gear.
So, the list you are looking at is an idealized situation. Rawr is telling you "Right now, in your current situation, THIS is better".
The 'list' will serve you better for planning "What is the FINAL gear set I am aiming for". Rawr will tell you "What will help me more RIGHT NOW"