That's normal for 480i. It's running at 60 fields per second, which is equal to 30 frames per second. Sometimes Windows reports that as 60Hz, sometimes 30, but it's still running at 60-fields/30Hz either way.
The card you are using has a TV-out port, and that port is locked to 480i over S-video.
EDIT: actually this is a bit weird. I guess Windows is actually detecting a monitor on that output? Or have you installed something like crt_emudriver? (you don't really need to use crt_emudriver with those TV-out ports, because they won't produce a great picture no matter what you do.)