I can duplicate the same behaviour on a dell laptop.
Ref $ cat ~/.xsession
Xvesa -br -screen 1024x768x32 -shadow -2button -mouse /dev/input/mice,5 -nolisten tcp -I >/dev/null 2>&1 &
and: $ cat /proc/bus/input/devices
...
I: Bus=0011 Vendor=0002 Product=0008 Version=0300
N: Name="AlpsPS/2 ALPS GlidePoint"
P: Phys=isa0060/serio1/input0
S: Sysfs=/devices/platform/i8042/serio1/input/input6
.. I used to be able to change the input to input6 (for example) and it would use only the touchpad and not the mouse, but this also doesn't appear to work.
This being said "mice" is meant to be a catchall.
@curaga?