$PS1 on Ubuntu VM does not match actual prompt

$PS1 on Ubuntu VM does not match actual prompt

I am running an Ubuntu VM out of Virtual Box and Vagrant:
vagrant@lucid64:~$ uname -a
Linux lucid64 2.6.32-38-server #83-Ubuntu SMP Wed Jan 4 11:26:59 UTC 2012
x86_64 GNU/Linux
My $PS1 variable doesn't actually match the prompt and is far more complex:
vagrant@lucid64:~$ echo $PS1
\[\e]0;\u@\h: \w\a\]${debian_chroot:+($debian_chroot)}\u@\h:\w\$
I am looking for an explanation of the relationship between the $PS1 and
my prompt, which is $USER@$HOST:$PWD\$, as can be seen above.
E.g., on my regular laptop, which hosts this VM and is a Mac, it is clean
and simple as follows:
~@12:10:20>echo $PS1
\w@\t>