It may be due to a common form of reference. You see the majority of “Taiwanese”, (and depending how you break down the terminology) their families at least can show common ancestry from China. Some “Taiwanese” can’t be traced back or show ancestry from elsewhere. Never the less, certain peoples of Taiwan call themselves “Taiwanese” regardless of fact, and others see themselves as relatives of Chinese. Those that find relevance to China refer to China as the mainland.
Its all rather silly really and it doesn’t matter how you refer to either place as it makes no difference at all which is worth noting. It only makes a difference if you think that somehow, a simple name has a real affect.
Personally I don’t know how names have affect, but I know how silly people put their lives on the line in order to maintain a simple name or indeed change a name to another name.
People without minds will often be found to actually sacrifice their lives in order to maintain a place name over another place name. It’s rather an old tradition that goes back quite a long way and is one of the reasons that people can’t progress a great deal further in life.