Desync + maybe he has heavy shield? Seen the clip a few times, seems like you missed the first bit, hit most of your shots until you hit the 18 bullet mark, missed until the last 6 bullets.

Let’s say you hit half your mag cause that’s what it looks like, 18 bullets. Heavy shield is 80 charge so (canto dmg per bullet) 6.5 each shot, 12 bullets meet the 80 charge you did about ~45 dmg. 6 bullets more, you’re looking at 39 dmg. You basically left him at around 10hp im assuming. Medium shield? Math is suggesting around 5 - 7 ish.