Is there a word for this?

Yes, phonology. In other words, it's how languages works. For example, hard as it may be to believe, the sounds represented by t in top, stop, and spot are all different (aspirated voiceless, unaspirated voiceless, and unreleased alveolar stops). There is a phoneme t in English that has several different realizations or allophones based upon considerations like position and context.

Ceci n'est pas un seing.