Not every paladin with more points in protection than any other tree would understand the subtleties of defence and being uncrittable.
I remember one guild I was talking to asking my what my resilience was when I was already uncrittable but working on uncrushable for Kara.
I replied that I did not need resilence as my defence was high enough. This was new information for the person I was speaking to though...
...and they were a paladin tank.
Not everyone automatically gets it without a whole lot of theorycrafting. The tooltips will tell you how much extra avoidance your defence gets, but it won't tell you how uncrittable you are. No where in the game itself - barring add ons, does it say:
"WARNING! CRITTABLE PERSON! GET MORE DEFENCE AND/OR RESILIENCE OR YOU HAVE A CHANCE OF TAKING DOUBLE DAMAGE HITS FROM MOBS!"
It's true the resilence tooltips will tell how much crit you avoid - but again, without theorycrafting, it's hard to know how much is enough, especially when you combine it with defence gear.
My apologies for the caps, but I am trying to make a point. As a general rule, I would as a healer ask a tank what their defence is, and then say they should get more if they are too low as that could help prevent big spikes that can kill them and wipe the group.
Being even more specific to the OP, it's likely the paladin tank was holy, won the sun-eater, and decided to go Prot to put it to good use. That would explain the poor itemisation and lack of defensive gear.
Please try and remember that paladin tanking stats are by no means obvious. We need a lot of different stats for a lot of different reasons, and there is no easy answer as to the "best stats" at any one point.
I would say as a rough guide though, get 485-490 defence, then get 10k health unbuffed, then get more avoidance until you are uncrushable, then get more health. That's how I use to think and it's gotten me pretty far.

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