Why do glow-in-the-dark substances dim gradually?
There are two potential questions here: one is why is dimming a slow drawn-out process (incoherent, unlike a "switch"). The second question is why different stars may appear to lose their intensity at a different rate. 

The light-emission process behind your stars involves phosphorescence. The process involves relaxation of an electronically excited system during which it undergoes a "forbidden" triplet$\rightarrow$ singlet transition. For an individual system, light emission is controlled by the probability of the quantum-mechanical interconversion (or equivalently by the lifetime of the triplet state). It is a stochastic event described by Poisson statistics or "shot" noise. 

However, the luminosity of one star represents the collective behavior of a large number of emitters (systems). One star, therefore, appears uniform in luminosity and its time-dependent luminosity decays exponentially. 

Variability might be expected in the luminosity of different stars. This is not likely to be QM statistics since, as already noted, you are observing a very large ensemble of relaxing molecules, and one fairly uniform decay constant is expected for all stars, assuming the phosphorescent material is uniform. It seems more likely that luminosity differences among or within stars may occur due to size effects or differences in the concentration of phosphorescent material, or both$^\daleth$. Stickers that are thicker or containing a greater concentration of excited phosphorescent material, due for instance to variability in the manufacturing process, should appear to glow longer simply because they contain more emitters$^\dagger$. 

$\dagger$ As helpfully pointed out in the commentaries, more emitters lead simply to the detection limit being reached later.

$\daleth$ As helpfully emphasized in yet another comment, and alluded to in the OP, it is the concentration of excited phosphorescent material that determines the luminosity, and excitation may also depend on the dimensions of the star.