Why did the "big three" disagree about the course of action to take at the peace conference?

The big three - Churchill, Stalin, and Roosevelt had different plans and maps on partitioning Germany and were to accomodate the fourth ally, France too. Finally they hammered out the plan that came into being. Poland with its shifting borders and territory was to be defined and the fate of East Prussia as well as Gdansk (Danzig) to be decided. It is a formidable array of negotiations with maps and stuff. All points were resolved and then implemented. It speaks very high of the co-ordination and responsibility of the allies that is seldom highlighted. Only the differences get highlighted. 
The major point of difference was the way to deal with Japan after the defeated that was imminent. Stalin assured that he will attack Japan within 90 days of the surrender of Japan as a revenge for its 1905 naval defeat. Nearly 9 out of 10 who were killed, maimed or missing in action were Soviets (Russians) for which there was no acknowledging from the other allies. After this heavy investment in blood and lives, Soviet USSR was prepared to have her last battle with Japan. After the death of Roosevelt Truman succeeded him, who pre-empted USSR move by dropping the only nuclear bombs to be used ever on Japan, without informing Russia as courtesy demands between allies. This highlights the exact relationships. Ideology and the reputation of Stalin as the bigger killer than Hitler suddenly occupied the centre-stage of the relations. From then on it was utter suspicion of what the other group (between the two camps of Russia and USA/UK/France) is upto that laid the foundation for Cold War that lasted from 1945 to 1990. Cold War is too big a subject to be dealt with here