In very old (really black and white, before sound and “talking pictures”) film, it was partly due to a number of competing standards for film playback and recording, or a lack thereof.
Another factor is an artifact of technology. Many cameras in early film did not have motors for exposing the film and were hand-wound with a crank. Thus the true frame rate was varied and depended on the steady hand movements of the cameraman. (In particularly amazing events of history one must watch in awe to consider how close a person had to stand and view these things and still keep a steady hand: such as the trenches of WWI, or Germany’s early rocketry experiments.)
Then these were played back via motor and showing the film at a constant framerate that was different than the recorded.
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