Again I disagree - a little. Video compression takes time, especially when there is a lot change between frames. And the reported frame rate might be only an approximation.
(Someone asked me what a Real Time Operating System was. And I didn’t know to laugh or cry. Per definition the real time system reacts within a given time. But it is idealistic definition. Well, from time to time I struggle with time available to process input signals. However I usually work with low pitch audio-like signals. Few hundred samples a second.)
There are two possibilities. The frame gets fragmented when the compression has not enough time. The other way is the real frame rate slows down, does not keep the recording rate constant.
vs