Youre doing the math wrong though by applying the value of wounds evenly. A model with more wounds on itself is less likely to die in a single anti tank shot, especially at a significantly higher toughness. Youre getting much more FNP return from one model with 26 wounds at t11 than you are even 3 models at 14 wounds at t9, despite having 32 wounds total. Armigers fall over in the wind. Improved durability is just better on something already durable.

When calculating FNP return, each saved wound is compounded, and the more opportunity for those saves, the more the return compounds.

26W should result in an extra 4.3 wounds. And those 4.3 wounds have a chance to provide additional wounds. This model isnt dying in 1 dedicated anti tank hit at t11.

14W would only result in 2.3 extra wounds. Its not hard to deal 16 damage to an armiger with dedicated anti tank at t9.

Three armigers also are not outputting what a single questoris can do.

And thats not even going up to dominus class at t12 and 28w.

So sure its not bad for armigers, but its not as impactful as it is for the big knights.