28.5.6.3 Byte NACKing

When the AHEN bit is set, CKP is cleared by hardware after the eighth falling edge of SCL for a received matching address byte. When the DHEN bit is set, CKP is cleared after the eighth falling edge of SCL for received data.

Stretching after the eighth falling edge of SCL allows the slave to look at the received address or data and decide if it wants to ACK the received data.