Textbook vs Real
Textbook정말요? 대단하네요!jeong-mal-yo dae-dan-ha-ne-yo
Real진짜? 대박!jin-jja dae-bak
Why Koreans say it this way
The K-drama double punch: 진짜 (really, casual) + 대박 (jackpot/amazing, slang). 정말 is the textbook word for "really" but 진짜 is 10x more common in real speech. 대박 replaced 대단하다 entirely in casual Korean.
From: K-Drama Common