I have worked for the Census Bureau during the past 2 census(es?) and
they do, in fact, mail out forms to every KNOWN address. If it is not
returned complete, they send a field agent to get the information(or
as much of it as possible).
The form you received could be been due to sudden local population
shifts, requested by your state, or other factors. If you have
questions, you should call your local government or the federal
government.