His American grandfather could have been in a US army unit that helped liberate one of the concentration camps in western Germany. That Obama said it was Auschwitz may just reflect his historical ignorance (since Auschwitz is in Poland and was liberated by the Red Army), rather than a deliberate lie.
Sometimes he is just indifferent to the truth, sometimes he is deliberately lying...it's all the same to him, since he doesn't care about the truth and neither does the media.