Ah I see. Actually, 之 in 之一 and 分之 represents the possessive relationship, although it might be a bit implicit. For example, 他是我喜欢的明星之一 “He’s one of the stars I like”. In this sentence, 明星之一 means “one of the stars” and 之 can be understood as “of”. Similarly, 十分之一 (one-tenth) means “one out of ten”, and you can also see 之 as “of”. I would say most of the 之 you see in modern context can be interpreted as “of”, and although it has many other meanings, you only see them in the traditional Chinese context (poems, literature etc.).
Could you think of other examples of 之 and where you saw it?