I'm sure someone can provide a better answer, and I can later when I get home, but the reason why Russian POWs were treated relatively worse by the Germans was partly racial and also due to the poor conditions of the Eastern Front.

Omer Bartov's The Eastern Front as a source: German propaganda emphasized the eastern war as a war between races, and specified the Slavic/Russian people as being 'sub-human.' German troops accepted the propaganda for varied reasons: it fell in line with their preconceived racial views; and in the miserable conditions of the front, they had to believe that what they were doing was important and framing the war as a racial battle with an incomprehensible enemy was convenient. Naturally, all of this led to poor conditions for POWs and the populace of Eastern Europe and Russia: the armies sustained themselves by excessive 'living off the land' (raping and pillaging), mistreatment of prisoners, executions of 'bandits' (either genuine partisans, Jews, or other Slavs. Some of the German commanders were concerned at the loss of discipline due to the encouragement of excessive violence, again not out of any sympathy, but out of fears of discipline.

The mistreatment of Russian POWs and the populations of Eastern Europe was due to the rhetoric framing the war as an 'epic battle between races with the Slavs as lesser and worse.' German soldiers largely accepted this rhetoric and acted accordingly due to their own preconceived notions or because it was a way to justify the grueling conditions of the front.

I'm on my phone right now so I can try to answer and/or expand on this when I get home. Source: Omer Bartov: The Eastern Front.