I do not know. I think, like anything else, it could change and people can change.
I am still fairly new to being involved in BDSM, yet I still considered myself to be a "SWITCH" from day one yet someone predominantly a sub more than anything else. (Degrees may vary from person to person that is a Switch. I do not know.) I do not doubt that someone could be 50/50 or even change, altogether, at some point. I think it happens (different reasons). 
I understand there is some contraversy within the BDSM community as to whether a "Switch" is really looked at with the same validity (?) [I just NOW learned that people have been ridiculed for having a D/s relationship, as well, and BDSM by others receiving flack. Why? BDSM or D/s may not be for everyone, as much as vanilla sex may not be for someone.] 
For me, I used to be more dominant (naturally, personality & sexually-speaking) before I was into BDSM. I wanted to please, though, even when young and even though I was a dominant Alpha-type personality. After certain events came in late teen/early adulthood, I became more submissive and desired that more, too. That is where the ability that I can be a Switch on rare occasions came in. It is not just to 'please' or as mood strikes. (I took some quiz on Lush BDSM that had me as a sub but borderline submissive who has a small percent domme... like 10% or something, and that seems to fit me enough).
As for those people who are unlike me and 50/50, or go from one to the other readily, all I can say is that some factors may pre-exist, others may be discovered, or life events may end up altering personal nature and personality. I have been one of those people to be a paradox most of my life. I am a double edged sword in most aspects. I am an introvert/extrovert, for example, so the thing about being a "Switch" comes naturally to me between two sides to me of past and present. I think it can be complicated.
All I know is I consider myself more of a sub and I identify there. That is what feels like I belong and most natural to me... HOWEVER, I will surprise my boyfriend now and then. We just watched Mr. & Mrs. Smith together with Brad Pitt & Angelina Jolie. (I do not know why I had not watched this sooner. I liked it!) We had a lot of fun watching the movie. They remind me of how we will wrestle and jocky for that dominant position in bed sometimes and when my dominant side comes out. I can be the Mistress to him, and he becomes putty, yet he often cannot take that for long until his predominant "Dominant/Dom" side takes over. It's hard for me to explain, but we have had fun with it. 
