Understanding group and passwd

Hi,

I am checking who belong to the dba group ,
and found that oracle and autosys users are part of this group
cat /etc/group | grep dba
dba::400:oracle,autosys

I thought to found user autosys under group 400 togther with user
oracle , but found it in group 1000 as you can see bellow.
cat /etc/passwd | grep -w 400
oracle::130:400::/software/oracle:/bin/ksh
extjob::131:400::/home/extjob:/sbin/sh
yoavb:*:119:400::/home/yoavb:/usr/bin/ksh

cat /etc/passwd | grep autosys
autosys:*:1000:1000::/home/autosys:/sbin/sh

Can one explain me where i am wrong ?
Thanks

Further to ctsgnb. It is not usual to itemise the membership of Primary Groups in /etc/group.
In your example the account "oracle" has Primary Group 400 and does not need to be listed in Group 400 in /etc/group . It is however harmless.