Here's one output of my 'onstat -g ses <sid>':

session #RSAM total used
id user tty pid hostname threads memory memory
77322 sampmeom - -1 samtst1. 1 6111232 783816
...
Memory pools count 1
name class addr totalsize freesize #allocfrag #freefrag
77322 V d3e64020 6111232 5327416 18414 2770

name free used name free used
overhead 0 1664 scb 0 96
opentable 0 4376 filetable 0 704
log 0 2152 temprec 0 6984
keys 0 1160 gentcb 0 1272
ostcb 0 2680 sort 0 56
sqscb 0 14248 sql 0 40
rdahead 0 160 hashfiletab 0 280
osenv 0 1216 sqtcb 0 3304
fragman 0 232 sapi 0 371696

I add up all used memory of memory pools, the value is 412320, but the 'used memory' for this session displayed is 783816. Should these 2 values be same?

What do you mean by:
I add up all used memory of memory pools, the value is 412320,

This is part of the virtual memory. You can't tune separate memory pools...