Jan 12, 2016
This could be where the "Braun" part of it comes in. Certain bloodlines might be able to use the serums to their full potential. In Chapter 27, Erwin says that the Female Titan has a similar ability, but that she can't maintain it like the Armored Titan can. Neither can Eren and we know he took Armored Braun serum. Eren and Annie also both show extra signs of fatigue when they use they use their "Armor" on top of their titan form.
Ah! Good point. I’m going to edit the original post and add this.