Why the grub boot loader is not installed to the first scsi drive (sda) when --driveorder option is not specified in the bootloader kickstart command

Issue

• I have a machine with two disks (sda and sdb), and I'm trying to install Red Hat Enterprise Linux 6.7 on that with a kickstart file.
• I could install it successfully on the almost machines, but it failed on some machines. It could not boot after installation.
• It seems that the grub boot loader was installed into sdb, and it caused the boot issue.
• If --driveorder option of bootloader kickstart command is specified, this problem never happens.
• But, I want to know the reason why sdb can be sometimes selected as a boot disk.