Why would interlacing happen when rendering from 1920x1080 to 720x480

I rendered an .mov from a Premiere CS6 project that is set for 1920x1080 and 23.976 fps. When I rendered it with the NTSC 24p widescreen setting to 720x480 I noticed a lot of interlacing that I did not see before in the project or on another render that I did with a setting of quicktime 1920x1080 with h.264 encoding with 23.976 frames per second. Is there any reason why I would notice interlacing now in the 720x480 version. I'm new to this, so any help is much appreciated. Thanks!