Dan Norton composed on 2017-12-23 19:15 (UTC-0500):
The menu inside the box is:
Debian GNU/Linux
Advanced options for Debian GNU/Linux
Debian GNU/Linux 8 (jessie) (on /dev/mapper/vol1-root)
Advanced options for Debian GNU/Linux 8 (jessie) (on /dev/mapper/vol1-root)
Debian GNU/Linux buster/sid (on /dev/mapper/vol3-root)
Advanced options for Debian GNU/Linux buster/sid (on /dev/mapper/vol3-root)
The first two boot stretch, so they will eventually have "9 (stretch)
(on /dev/mapper/vol2-root)" appended, once the timeout is under control.
Based on what I see and what you say, it seems you are modifying the timeout for
Stretch (/etc/default/grub on vol2), but actually booting Stretch from Jessie's
grub.cfg (/etc/default/grub on vol1), which remains configured to 3 seconds.