Comment 6 for bug 1046627

Looking at the partman logs:
Before state:
parted_server: OUT: 1 1048576-2047868927 2046820352 primary unknown /dev/sda1
parted_server: OUT: 5 2048917504-12548308991 10499391488 logical ext4 /dev/sda5
parted_server: OUT: 6 12549357568-12748587007 199229440 logical ext4 /dev/sda6
parted_server: OUT: 7 12749635584-52748615679 39998980096 logical ext4 /dev/sda7
parted_server: OUT: -1 52748615680-160041885695 107293270016 pri/log free /dev/sda-1

The last bit shows that there was ~ 10GB of empty disk space available.

Resize_use_free auto partition mode was available:
/lib/partman/automatically_partition/10resize_use_free/choices: Found resizable partition '/var/lib/partman/devices/=dev=sda//12749635584-52748615679' (/dev/sda7) with 10968731648 bytes free

The end result became:
parted_server: OUT: 1 1048576-2047868927 2046820352 primary unknown /dev/sda1
parted_server: OUT: 5 2048917504-12548308991 10499391488 logical ext4 /dev/sda5
parted_server: OUT: 6 12549357568-12748587007 199229440 logical ext4 /dev/sda6
parted_server: OUT: 7 12749635584-52748615679 39998980096 logical ext4 /dev/sda7
parted_server: OUT: 8 52749664256-157895622655 105145958400 logical ext4 /dev/sda8
parted_server: OUT: 9 157896671232-160041009151 2144337920 logical linux-swap /dev/sda9

So indeed biggest free space was used, which is expected behaviour.