Plausible explanation: there's obviously a ricochet off the pool score tab right before Walt winces. a round ricochets up to the score piece, off that, off something else, maybe the wall or the floor, into Walt's side. It's not a "mortal" wound at that point, but neither he, nor Jesse do anything to stop the bleeding. Walt doesn't care at that point. Several minutes go by finishing off Jack, talking to Lydia, etc. those two exchanges with Jesse. It looks like he just laid down too, not like he collapsed. Several more minutes would have gone by before the police get there and by then it's all over now, baby blue.
Bigger question is what would have happened if he wasn't wounded?