See take that 400 hit you advertised with. The difference between wearing the plate mail (total armor 100 hardiness 100%) and wearing the robe (total armor 75 and hardiness 30%) is then 400-100 x multipliers or 400-75x multipliers. Even with a 4X multiplier you are only looking at 100 more damage.
200. Not 400.
100/(50+) against 200 is 100 impact, or halving the base damage. 75/30 is going to come out to 60, or 140 remaining damage getting through. Meaning you take something along the lines of a third more damage, which is non-trivial. You are looking at 160 more damage, which, on a hit that's doing 400 vs 560, is pretty notable.
However, against 150-which is more common, many standard physical fighter enemies can pull this, 200+ is only the very hardest hitters in general-100/70% still caps out at 100. Dropping the damage by 2/3rds. 75/30 will block 45-dropping the damage by one third. It's the difference, with the same multiplier of 4x involved, between taking 200 and 420.
In other words, you take twice as much damage with light armor in this case, as with heavy, and the returns increase the lower the damage gets-meaning that heavy armor is far better at keeping you from getting mobbed.
You essentially are also ignoring that heavy armors have better egos and artifacts, in general, than light armor. While Light Armor finally got some okay elemental spread defense egos relatively recently, there is nothing in ego light armor that matches Hardened or Dragon egos, and there's only one light armor that really competes with the normal elemental defense armors...and it was specifically designed to mostly be an AM thing, with bad physical defensive abilities relative to its tier, particularly without the AM bonus.
This is because most of your armor comes from all your other item slots and hits from creatures that matter take into account all your armor anyway even at a hardiness of 30% for that robe. Or they IGNORE it anyway.
Nothing much fully ignores armor.
Enemies do have APR, of course, but APR means that the 30~ or so extra armor you'll get from heavy armor matters more, not less. Let's look at the 150 example, and assume the guy has 30 APR.
In the former case, 70/70 of the armor/armor hardiness, you'll block 70, down to 130 base, or 520. Nasty hit.
In the latter case, though, you'll have 40/30, which even with Armor Hardiness kicking in before to lower your armor, means you still take 160-or 640.
Yes, on paper, the returns diminish...but there's more than one way to look at a statistic like this. In both cases, you're getting closer and closer, with nasty hits like this, to getting twoshotted. Two hit kills are huge dangers-you can't do anything but blow resources and die if you match them turn for turn(unless you are very, very confident that you oneshot the enemy back), and if the enemy has a speed bonus you'll get effectively one-shotted. The difference between getting killed in two, and in three, hits is quite large. You don't want to get any closer to that than you have to.
It means more than you'd think, in other words.
Also, another thing to note is the critical hit rate drop.
Now, I know what you're thinking-most people are thinking-when they look at that. 15% or so, 20% or so, isn't a large drop-some enemies can critical all the time.
But many enemies have low-single digit-critical hit rates...but getting critted by them still means sudden, nasty damage. An example of this I find very prominent is Armored Skeleton Warriors. They hit pretty hard on base-a critical can mean the difference between surviving but being in pain from a technique from one, to being dead, and the Armor Training margin prevents this from ever happening. Not so with light armors.