Why does the letter “x” in “fox” represent two phonemes?

The letter "x" in "fox" represents two phonemes because English orthography does not always maintain a one-to-one correspondence between written symbols (graphemes) and distinct speech sounds (phonemes).

In the case of "fox," the single grapheme "x" functions as a shorthand for the consonant cluster /ks/, representing the /k/ and /s/ sounds produced in sequence. This is a common feature of English, where individual symbols can represent multiple phonemes, just as other symbols or series of symbols may be pronounced in various ways depending on their context within a word. Because standard writing systems often fail to account for the complexities of spoken language, phonetic transcription is used by linguists and in dictionaries to clarify the actual pronunciation of such words.1