Unit in Accumulator in the Particle Tracing

Hi, all:

I would like know the particle distribution in a 3D domain, so I added "Accumulator" in the Charged Particle Tracing. I am wonder about the unit in the Results (Particle Trajectories, Particle Trajectories, Color Expression, "Accumulated variable name: rpd"). The units for "cpt.rpd" were both "1," regardless "Count" or "Density" was set in "Accumulator." I had checked the values, "Density" = "Count" / "meshvol" was correct. Since Density = Count / meshvol, the units of rpd should be [1/m^3] and [1] for "Density" and "Count", respectively. Is it a bug of COMSOL?