The thread on a flare fitting is a 'running' thread, like a bolt. Running threads are also found on compression fittings, slip-joints and the converge of unions.
Iron pipe threads are tapered, meaning they are smaller at the tip of the thread. The corresponding fittings are inversely tapered, meaning they are larger nearest the opening. These fittings seal by interference, meaning the male threads on the pipe mesh with the female threads of the fitting.
Running threads have no sealing properties. They are merely there to help 'sandwitch' the pipe to a ground joint connection.
Running threads and tapered/ips threads are incompatible with each other.