This answer applies to the Tau 320, Tau2 324 and Tau2 336 cameras, as well as the Quark 336 and Quark2 336. NTSC is an interlaced analog format. The 320 x 240 image is sent twice, once for each field. This provides 480 lines.
Each line is output in the standard NTSC line time so that (except for any display overscan) all pixels are represented.