  Recently Pop ({\it Solar Phys.} {\bf 276}, 351, 2012) identified a Laplace
(or double exponential) distribution in the number of days with a given
absolute value in the change over a day, in sunspot number, for days on which
the sunspot number does change. We show this phenomenological rule has a
physical origin attributable to sunspot formation, evolution, and decay, rather
than being due to the changes in sunspot number caused by groups rotating onto
and off the visible disc. We also demonstrate a simple method to simulate daily
sunspot numbers over a solar cycle using the \cite{2012SoPh..276..351P} result,
together with a model for the cycle variation in the mean sunspot number. The
procedure is applied to three recent solar cycles. We check that the simulated
sunspot numbers reproduce the observed distribution of daily changes over those
cycles.
