That is because you happen to be ignorant of all but the most notable thing that could be either "xi'an" or "xian". Others are not limited by the extent of your knowledge.
Virtually 100% of English speakers are (in fact, I'd go so far as to say nearly 100% of english speakers are ignorant of all the notable things that might be xian or xi'an). Of the remainder, none will be confused (your worry is ambiguity, right?) by the sort of reference we see here.