All guides are based on set assumptions. Veneretio's guide is explaining which items are best assuming a certain baseline of gear. If you walk into this gear guide and have blues everywhere but a GREEN chest, of course your top gear upgrade would then be t7 chest, but then I'd ask you why haven't you obtained a better chest first?
It's all subjective, this is just a rough guide as to which pieces generally are better based on a set naxx progression tank.
These are all "GUIDES".... not "THOU SHALT GEAR YOUR TOON THIS WAY". Read through them, and spend some time learning the mechanics of the game so that you can make educated, logical decisions. Gear is always changing. You need to have an understanding at to what your class's key stats and abilities are and build to support a concept, be it dps, healing, or tanking.
And please, do not get seduced by "gear score". It counts for nothing if you have mismatched gear that conflicts in its purpose, or doesn't support the talent build of your toon.