juju deploy --to lxc:0 cs:trusty/ubuntu creates precise container

I have a precise bootstrap node, and I run the following to test a trusty charm:

juju deploy --to lxc:0 cs:trusty/ubuntu

And I see the following on the bootstrap host:

 16318 ? S 0:00 \_ lxc-create -n juju-machine-0-lxc-0 -t ubuntu-cloud -f /var/lib/juju/containers/juju-machine-0-lxc-0/lxc.conf -- --debug --userdata /var/lib/juju/containers/juju-machine-0-lxc-0/cloud-init --hostid juju-machine-0-lxc-0 -r precise

Notice the '-r precise'

Indeed, I end up with a precise container, and juju status output that still shows "trusty/ubuntu":

machines:
  "0":
    agent-state: started
    agent-version: 1.17.7
    dns-name: xxx
    instance-id: i-yyyyyy
    instance-state: running
    series: precise
    containers:
      0/lxc/0:
        agent-state: started
        agent-version: 1.17.7
        dns-name: 10.0.3.120
        instance-id: juju-machine-0-lxc-0
        series: trusty
        hardware: arch=amd64
    hardware: arch=amd64 cpu-cores=1 cpu-power=100 mem=1740M root-disk=8192M
(...)
  ubuntu:
    charm: cs:trusty/ubuntu-0
    exposed: false
    units:
      ubuntu/0:
        agent-state: started
        agent-version: 1.17.7
        machine: 0/lxc/0
        public-address: 10.0.3.120