Well, they never will add up to 103% by themselves. There was of course a distinct cap where you would be crit immune at 440 defense, which required 140 defense from gear, 130 if specced in Anticipation, but for most of vanilla, going defcap was an active choice you made since not all gear combos you needed to have would provide enough defense, so either you ran with your defcap set or you ran with some other type of set, maybe resistance or threat. Most tank epics would have somewhere between 5-7 defense on them each (so for example if you had pieces with 7 defense in each of the 17 possible equipment slots you'd still only reach 119 extra defense from gear). But just getting defcapped didn't mean you could avoid crushing blows, which is what the 103% total avoidance is supposed to do. However, there's no way to reach that avoidance from just stats on gear. The defense itself would not be part of the 103% equation, but rather the avoidance that each point of defense provided (a small amount of dodge and parry for each point of defense, so defense aquired beyond the 440 crit immune cap would still add to your total avoidance). Most tanks will hover at somewhere around 15%-ish for parry, dodge or block each, so anywhere between 40-45% total avoidance, and that's with raid gear, while defcapped. But like I said before in this thread, Warriors have the Shield Block ability which adds 100% block chance and can almost be on a 100% uptime, so therefore, Warriors could go beyond the 103% avoidance point. Blocking an attack would still be the "worst" way to avoid a crushing blow however, since you would still take a majority of the damage from the attack, so parry and dodge, which completely negated the damage, were still better and in no way useless just because a Warrior could easily go above 103% avoidance most of the time.
So no, I wouldn't say tanks had the easiest time picking what gear to use. Just slapping on 8/8 Wrath wouldn't necessarily net the best results.