Blau highlights use of two kinds of letters for _chet_.
Joshua Blau, “On Polyphany in Biblical Hebrew,” Proceedings of the Israel Academy of Sciences and Humanities 6, no. 1 (1982): 144
On principle, this may be interpreted as an attempt to substitute for ḥ, absent in Greek, what to Greek ears seemed to be the closest representation. ḥ was perceived to be a sound between zero/vowel and χ, and therefore, it was, in an impressionistic way, sometimes transliterated by zero/vowel mutation and in other cases by χ.