Wave/particle duality in Young-slit experiment

The animation above shows a simulation of the Young-slit experiment, performed with either photons, atoms or electrons. In this experiment, the rate is low enough so that you can observe the particle impacts one by one.

You can change the number of Young fringes as well as the particle rate.

Can you explain, both qualitatively and quantitatively, why the relative noise in the histogram decreases during the accumulation ?