PS shows different RSS value from Pmap and Smem

I want to get the memory consumption by a process. I am using *RSS* parameter to determine it. I am getting same *RSS* value when I am using pmap -xx and with smem tool. But when I am using ps v to get the RSS then it is showing different value. I know for pmap and smem they use /proc/pid/smaps file to get *RSS* value but I don't have idea about ps tool. What are the basis of this calculation in ps tool and which is correct ps or pmap and smem