What resource are you using for the comparison listing to Zillow? Trulia listings, MLS, tax records? Is the Zillow information an actual listing, or the general Zillow info on a property?
If you could post links for comparison I would be of much more help to you.
The county record is sometimes inaccurate in room count on single family homes, especially in Arlington, but it sounds like you are looking at condo listings. Is that correct?
Basement rooms can be included as bedrooms as long as there is appropriate egress. Your loft question does touch on a gray area in terms of how to represent the unit in the most accurate way possible. You could really argue for or against either angle. Especially gray is the "Studio Loft". Is it a studio unit with a loft, or a Loft style studio?
At the end of the day Virginia is a "let the buyer beware" state, so it is up to the buyer to perform their due diligence prior to purchasing a home. You want to see pictures, view the property, etc, so that you can make your own determinations.