How does loot work on TBC Blackrock?

Hello all, curious about the T5 Quest Vendor and the Weekly quest chains specifically if you do the chain that starts off with 1 arena win, then Eye of Storm flag cap, then 5 kills within 15 seconds, etc, do you get better chance at loot the further you continue the quest chain? or is loot chance purely random? I'm a hunter and I've seen a couple people with 3 pieces of the new T5 gear and the new bow and was wondering how they get it? do you get better loot further down the quest chains is main question, appreciate it, I also noticed the 2nd questline that has roughly 15 quests where you turn in 5 bg marks at a time gives you a lot of ilevel 115 gear but then about 5 of those boxes give the higher 125-130ish newer gear is that all the same drop rate chance as the other questline? just want to make sure im getting the best possible odds of getting better gear to upgrade, thanks a bunch for help!

You get gear from corresponding to quest raid boss (quest description says wich boss you are "doing"). Its not getting any higher chance of loot while quests continue. The only thing to mention is that you get Nether Vortex 100% from the last quest in chain (Kael'thas one)