Volumetric E question observasion (M200)

Besides using Max Volumetric E as described in the previous post we use Volumetric E in the printer setting main reason was as we sometimes had access to a printer that ran on 2.85mm filament now if you select Volumetric E in the printer section of SLIC3R/Prusa slicer the slicer calculates the material in mm3 (volume) instead of mm (length) you then either select the filament diameter by sending
M200 D1.73
to the printer or D1.75 or D2.85 whatever you measured the filament at and if the printer can handle M200 (all marlin based can) then it will properly print that now its neat that you only have to make one Gcode file and the operator always has to select the filament or set it as a default or macro or whatever is applicable

Now the observation/question is that we seem to have gotten better prints since going volumetric in particular if you lets say print a 2mm wall that is 100mm long or so there is less of a thickness change between the ends and the starts and the middle. So we are doing it all the time now

Question is why? or did something else cause this as when we found the need to do that we switched to SLIC3R and also instituted the Max volumetric E plus well using SLIC3R so anyone know if Volumetric E (M200) does something inside marling that M200 D0 does not?