Spoken languages do not follow rules, it is the other way round, rules are written in an attempt to explain spoken language from a pedagogical perspective. Speaking about the letter 'Chi' we are judging its phonetic value from modern Greek, however, we are not entirely sure how it sounded in Ancient or Koine Greek. If an alphabet is shared between two languages, it does not mean that one owns the 'correct' pronunciation and the other is a mutilated copy. It is just that phonetic values vary from a place to another akin to the Latin script which is used in many romance languages yet, many letters eg a, b, c, e, h, j, n, o, p, r, s, t, u, x, etc. The same with Arabic abjadeya where the pronunciation of certain letters vary within Arabic in different places and in comparison with Persian and Urdu.