I speculate that sometimes the steadfast character appears to change because:
1. He has a "Buzz Lightyear moment" during which he temporarily adopts that character's solution but then reverts back to the original problem/drive (perhaps with a different formulation or within a different context).
2. He overcomes his critical flaw.
3. He learns, experiences, or does something life-changing. Life change isn't Dramatica Change. Captain America didn't turn his back on his starting drive or ideals; he turned his back on a wrongheaded person/mission (vague for no spoilers).
4. He changes the means of reaching an end, but the end is the same. That is, he differently scratches the same itch.
5. Last ditch effort to keep the Change character steadfast: The Change character changes so much in a non-leap of faith way that the steadfast character's reaction to their finally recognizing the Change appears to be a change in the steadfast character due the sudden outburst. Kate in the Godfather. Truby insists Kate changes at the end. She reacts to Michael's change by setting boundaries.
In all these cases, I believe that the character still remains steadfast because their problem/drive is retained at the end when compared to the beginning (i,e. the solution isn't adopted at the end). To quote the end of the definition of change in the theory book, "When a Main Character abandons his original story-long approach for its counterpart, he is said to Change."