By analogy, any two watches are nominally running at the same rate, clocking up 60 seconds every minute.
In order to transport multiple data streams from one place to another over a common transmission medium, they are multiplexed in groups of four.
This extra gap is in a fixed place in the frame and is referred to as the "stuffable bit".
The process can then be reversed by the demultiplexer and 4 data streams produced with exactly the same bit rate as previously.
Therefore, the stuffing ratio is carefully chosen to give theoretical minimum jitter.
The position in the frame when the decision is made varies and adds a second variable dependent on the length of the store.
Variable storage buffers, installed to accommodate variations in transmission delay between nodes, are made large enough to accommodate small time (phase) departures among the nodal clocks that control transmission.