Restaurants typically hold back inventory from online reservation systems for two reasons. They either think they can fill seats and they rather not pay the per cover fees OT and UrbanSpoon charge or they want to hold seats for walk-ins. Well three if you count timing seating to keep the front of the house and kitchen from being overloaded.
State Bird Provisions however has so much demand that there is a line for walk-ins hoping for a cancellation an hour before the restaurant opens and they appear to not actually hold back much (any?) inventory specifically for walk-ins.
But generally speaking yes, you will often be able to call a restaurant and get a table or possibly walk in even when their online reservation system shows up as full.
Also, you can also put yourself on a waitlist at many restaurants. If someone calls to cancel, before they release the reservation back to the online system, they'll go through their waitlist and call and ask if you want it instead.
From their site: "We welcome walk-ins on a first-come, first-served basis and set aside about a third of our seats for those guests every night, including our chef’s counter and several tables in the dining room. "