Well, while V.V is not wrong about voicelessness, there's actually something else out there. What you seem to hear is actually an evidence of so called "явления придыхательности глухих согласных" (aspiration of voiceless consonant). It's not х it's more close to the phonetic [h] (like in English hen). In some cases (usually before other voiceless consonant) in casual speech there consonant can gain some level of aspiration.
For instance, pronounce following words and take attention to how they sound (the potentially aspirated vowel is marked):
As of why it's not indicated somehow in spelling - well, there's not such thing like 100% precise, one-to-one correspondence between phonetics and orthography. One reason for that - some of phonemes a very specific, some a relatively rare, some a very subtle (I doubt majority of Russian speakers even realize they are aspirating anything).
Also, if you'll try nevertheless to come up with "precise" orthography you'll fail for both because words are not pronounced the same very way by all the speakers, second, pronunciation changes with time.
If you are interested to find out more, check out, for instance, this article.