I think where armies are undisciplined (no punishment), or respect for the non-combatant or combatant is non-existent many will do what they want.
The worst rapists in a modern army were probably the Russians after the fall of Germany. I'm sure the Germans did some nasty things. The Japanese were pretty horrible.
Have you ever known an army, even through history, which made a difference between combatant and non-combatant ? Egyptians wiped out entire tribes, so did the greek, persians, ...
In modern war the line is drawn somewhere, but easily crossed, eg the bombings in afghanistan don't only hit combatants.
Who are the biggest rapist is a biased opinion. You think US soldiers never did anything wrong in Vietnam ? Or the Khmer ?
The figures are never correct and will never be.