Disclaimer: I don't know anything about Truby, so my interpretation is limited to your description. That being said, as I explained in another thread, the difference is in those ideas of "Steadfast" and "Change." Steadfast characters are allowed to learn; they're allowed to struggle; they're allowed to grow and develop. They're just not allowed to change their fundamental resolve. The Relationship arc (and to a lesser extent, the Overall arc) causes the Steadfast character to learn and mutate.
Let me try and explain with an example. In this one episode of my favorite show, a troop of small, delicate creatures (fairies, basically) get temporarily split off from their friends. They're weak and defenseless, so while they're recuperating, one of the main characters takes them in and shelters them. She wants to coddle them and let them stay as long as they like, but their captain insists that she needs to be firm and kick them out so they can get home before the portal home vanishes. However, because his insistence is so brusque and derogatory, nobody ever listens to him. In the climactic scene in the episode, the main character realizes that she must do the right thing and kick them out, even if it feels unkind, while the captain realizes that he needs to be gentler and more supportive if he wants his troop to listen to him.
This sounds like a story with a compromise, but it's really not. While both characters learn a lesson, the main character fundamentally changes her attitude towards kindness and support, whereas the captain fundamentally remains steadfast; he only changes his behavior. In the end, the captain is right, and the main character changes to be more like him.
This is incredibly important to Dramatica theory because it boils the story down to a fundamental problem: Support vs. Oppose. The main character struggles to change because she confuses Oppose with Hinder, when in reality she can Oppose while still being helpful. On the other hand, while the captain does have some back-and-forth regarding Faith and Disbelief, he never fundamentally abandons his desire to Pursue the portal. (EDIT: I plugged in the storyform for this, and it ended up that the MC Critical Flaw was Responsibility. Perfect, right?! That makes the IC Critical Flaw Morality, but I think this makes sense if you take it as lack of Morality. Because the captain has no concept of treating others with respect, he runs roughshod over them, which makes it difficult for him to be heard. But it's his Commitment to reaching the portal that leads to the MC changing. [And the RS Catalyst is even Delay! I love Dramatica's intuition!])