They're related. As you know, Ш represents a voiceless fricative sound very similar to the SH in English. Ч (and the English CH) generally represents an affricate. Affricative consonants are just fricative sounds preceded by a plosive/"stop". In this case, what's basically a T sound crammed up against a SH sound.
It's not surprising there's overlap. Similar things happen in many languages.