It's likely due to different methods of upscaling the picture frequency.
Early movies had 16 pictures per second1, which can be a problem converting to the 25(PAL) or 30(NTSC) per second needed for TV.
That's not a nice simple ratio and doing something like simply doubling each picture except for every 8th will give 30 per second, but smooth movement will have perceivable jerks twice a second where a picture is shown half as long as the rest.
1) with lots of variation due to the earliest hand cranking and later spring driven.
Reductionist and proud of it.
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