15.6+- khz is the time it takes to produce a single line on a NTSC tv, so the apple screen is probably really running at or near ntsc tv specs
Also it was very common in the 80's to use parts from tv's and radios, since they were the most common and most produced electronics at that time (the apple II uses a NTSC color burst crystal 3.58mhz)

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The master clock is 15.x MHz, which is not the same as the horizontal line rate (kHz). The latter works out to something like 22.x kHz in the monochrome compacts. The ~50% higher line frequency makes interfacing to an NTSC device very difficult.

The Plus does not use any frequency related to the NTSC subcarrier frequency. The clock frequency is a consequence of several compromises among screen resolution, CPU speed, and the need to support a standard set of baud rates.