In my early years of learning about the Enneagram types, book after book I read would pretty much say the same thing regarding my Enneagram type. They would make a brief mention at the beginning of the book that I'm actually all nine types, but then go on to discuss how I'm only one of the nine types. This seeming contradiction was never clearly explained in anything I read or heard about regarding the types. Since my goal from the beginning was to discover all nine types within myself, I had to come up with my own understanding that explained that contradiction.
What I found is that there are two components to Enneagram type. There's the focus of each type and there's the response to that focus.
All nine types of focus are available to each of us. We do shift to different types of focus on a regular basis. However, we tend to discount certain types of focus as not being relevant or important. In so doing, we tend not to act upon those. On occasion, we may actually respond against those types of focus in an attempt to counter them in favor of a focus we do feel more relevant and important. This is the dynamic at work underneath personality between the nine types within us. Unfortunately, this hasn't been much explored nor addressed in the traditional Enneagram type material as yet.
What most people refer to as my Enneagram type, I refer to as my dominant Enneagram type. I do this to emphasize that I"m really not a single Enneagram type. It's just that one of the Enneagram types dominates over the others. It's this dominant Enneagram type that is most seen in personality because it's the type of focus that is deemed most relevant and important. It has the tendency to keep other types of focus from being expressed not only because it tends to silence those other types of focus by overriding them but also because we know how to respond from our dominant type of focus since we have plenty of practice and experience doing so. We have much less experience responding from the other types of focus in comparison. This very often leads to a tendency not to respond from any of these other types of focus, even when it may be obvious that it would be a more appropriate response at the time.
If point 9 is the ideal unification of the ego, why would another perspective dominate?
Sounds like you're asking me to justify an assumption I don't have. Maybe your answer lies in asking the question "what is the dominant focus of type 9 and how does that differ from the other types?"