Bhagavad Gita
4.6

अजोऽपि सन्नव्ययात्मा भूतानामीश्वरोऽपि सन् | प्रकृतिं स्वामधिष्ठाय सम्भवाम्यात्ममायया ||६||

ajo'pi sann avyayātmā bhūtānām īśhvaro'pi san prakṛtiṁ svām adhiṣhṭhāya sambhavāmy ātma-māyayā

Translation

Although I am unborn and My transcendental body never deteriorates, and although I am the Lord of all living entities, I still appear in every millennium in My original transcendental form.

Interpretation

Krishna's incarnations are different from ordinary rebirth. He is 'aja' (unborn) — he has no karma forcing him to take birth. He appears through his own divine power ('ātma-māyayā' — his own creative will/maya), not through the mechanism of karma that drives ordinary rebirth. His body does not deteriorate; it is transcendental.