Some frame numbers are being repeated

I am using QuickTime Player 7 Pro. I have Super 8 movie transfers. They are at 18 fps. The frame counter repeats some numbers according to the following pattern:

Each second of time codes looks like this:

17:00:49:00

17:00:49:01

17:00:49:02

17:00:49:03

17:00:49:04

17:00:49:05

17:00:49:00

17:00:49:01

17:00:49:02

17:00:49:03

17:00:49:04

17:00:49:05

17:00:49:06

17:00:49:07

:

17:00:49:15

17:00:49:16

17:00:49:17

17:00:50:00

etc.

Why are the first six numbers of each second repeated? What is the purpose of this? Why would anyone want it this way? Is there some way renumber the frames without the repeats? I tried various Timecode settings, but to no avail.

A similar thing happens with the frame numbers. It seems to be trying to make each second contain 24 frames, but at the expense of repeating some of the frame numbers. Why in blazes is it doing this, and how can I fix it?

Thanks!