RSS(resident set size) is differ when use pmap and ps command

 
$pmap -x 10400 10400: ./a.out Address Kbytes RSS Dirty Mode Mapping 0000000000400000 0 0 0 r-x-- a.out 0000000000600000 0 4 4 r---- a.out 0000000000601000 0 4 4 rw--- a.out 00007f957085f000 0 4 0 r-x-- libc-2.15.so 00007f9570a12000 0 0 0 ----- libc-2.15.so 00007f9570c11000 0 16 16 r---- libc-2.15.so 00007f9570c15000 0 8 8 rw--- libc-2.15.so 00007f9570c17000 0 8 8 rw--- [ anon ] 00007f9570c1c000 0 0 0 r-x-- ld-2.15.so 00007f9570e1f000 0 12 12 rw--- [ anon ] 00007f9570e3c000 0 8 8 rw--- [ anon ] 00007f9570e3e000 0 4 4 r---- ld-2.15.so 00007f9570e3f000 0 8 8 rw--- ld-2.15.so 00007fff18a6d000 0 12 12 rw--- [ stack ] 00007fff18bff000 0 4 0 r-x-- [ anon ] ffffffffff600000 0 0 0 r-x-- [ anon ] ---------------- ------ ------ ------ total kB 4148 92 84   RSS is 92  $ ps -o rss,sz,vsz 10400
80  1037   4148



why and what happened ?

   The SIZE and RSS fields don’t count some parts of a process including