Film Inception, why is that the totem was still spinning?

I can't understand why Mr. Cobb starring Leonardo was still in the form of a dream while it was simply stated that he was in a reality for he claimed his reward of going back home to be with his children? Was there any mistake on the process to be in a dream there? Is it really hard for Mr. Cobb to distinguished whether he is in a reality state or just simply dreaming? It makes me confused that why in the end of the film, the totem were still spinning? I thought it was just completely a reality already ... tnx