Why one-to-many mapping between IPA and Pinyin? I am referring to the Origin of zhuyin symbols table on Wikipedia. Under Rhymes and Medials the following 3 entries confuse me:




Zhuyin
IPA
Pinyin




ㄧ
i
i/y


ㄨ
u
u/w


ㄩ
y
ü/yu/u




Why are there multiple Pinyin symbols for the same IPA sound? Are there any general rules as to which one of these symbols should be preferred?
You can find the explanation in page for Pinyin on wikipedia:

Note on y and w
y and w are equivalent to the semivowel medials i, u, and ü (see below). They are spelled differently when there is no initial consonant in order to mark a new syllable: fanguan is fan-guan, while fangwan is fang-wan (and equivalent to fang-uan). With this convention, an apostrophe only needs to be used to mark an initial a, e, or o: Xi'an (two syllables: [ɕi.an]) vs. xian (one syllable: [ɕi̯ɛn]). In addition, y and w are added to fully vocalic i, u, and ü when these occur without an initial consonant, so that they are written yi, wu, and yu. Some Mandarin speakers do pronounce a [j] or [w] sound at the beginning of such words—that is, yi [i] or [ji], wu [u] or [wu], yu [y] or [ɥy],—so this is an intuitive convention. See below for a few finals which are abbreviated after a consonant plus w/u or y/i medial: wen → C+un, wei → C+ui, weng → C+ong, and you → C+iu.

Well, this note is so detailed, I really feel it difficult to add something… maybe it can be concluded as this:
When [i] and [u] sounds appear at the beginning of a Chinese character, they're written as y and w, in other cases they're written as i and u.