Twice further the target, four times larger is the area filled with bullets. So the "bullet density" or "bullet flux" is not the same, but it is four times lower. And with the same sized target, you have accordingly lower probability of hit at greater distance. Simple, isn't it? :-)
Plus the bullet flight time is double.
While the SRP is calculating the firing solution assuming straight target path, our targets are usually turning.
They have time to turn away further from the calculated impact point, as the Tu is longer.
