backup of my database and my files have the secondary group

Hello,

      I execute a backup of my database by rman with the user of my database and the files of my backupset have the secondary group asmadmin.

>>>id wincnprd

id uid=502(wincnprd) gid=1001(oinstall) groups=1001(oinstall),1002(dba),1004(asmdba), 1006(asmadmin)

>>>

RMAN target targdba/targdba_passwd@ORACLEDB rcvcat rman_dba/rman_passwd@RCAT

RMAN >

RUN {

ALLOCATE CHANNEL ch00 TYPE DISK;

BACKUP

    # recommended format

    FORMAT '/oraexport/WTPRDADM02V01/RMANBKP_WINCNPRD/bk_%s_%p_%t'

    DATABASE PLUS ARCHIVELOG;

sql 'alter system archive log current';

RELEASE CHANNEL ch00;

}

>>> After my backup my file have the group asmadmin which is secondary.

Then i do not understand why is not the primary group 'oinstall'