Maybe it has something to do with when you throw I ions in the solution it's not like you add them in a +2X fashion. But when I combined with Pb it's gonna be 2I's for every 1 Pb. I think it's pretty common (haha) for common ion effect problems. Basically you don't have the coefficients when you do the ice table because the ions aren't dissociating in a set fashion. You're just throwing ions in the solution which then they combine in an orderly fashion (for example 2 I's for every Pb) to form the precipitate. maybe someone else can add something. Orgoman22 would definitely be your best bet haha