It is a rate that is market, and is adjusted annually. It is not the best rate but close enough for tax reason.
It's not close enough, especially for taxes. How does it differentiate, for example, between a single-family detached house, a town/rowhouse, and an apartment/condo, other than by the number of bedrooms? Each of these types of properties, even if they are all 3 bedrooms, will have a very different fair market rent.
IRS can also check the newspaper for the "market" rate. That's not too hard in an audit.
That's not a market rate because no contract has yet been signed on that particular property at the advertised price. The contract may well be signed for a lower price.
Do you think your city assessor just opens the classifieds to figure out how much your house is worth, or does he look at actual sales? Same idea.