It’s Onyomi (The Chinese-based phonetic way) vs. Kunyomi (the Japanese own phonetic way) of pronunciation.

Like 心 kokoro vs. 心臓 shinzou. The latter in simplified Chinese that this is based off of is 心脏 (Xinzang), which sounds similar.

Commonly, Onyomi is used when multiple kanji are used to describe a single “word” or concept, and Kunyomi is often used when on its own or is a verb with its own trailing character conjugation (okurigana).

Many exceptions apply but I hope this rule of thumb helps you.