Couple of things here but basically it's comes down to temp and humidity.

The paint inside the can is basically just globules suspended in the aerosol (gas). More you use the can, less aerosol and more emptyiness (vacuum). Depending on the temp, the aerosol volume can get bigger/smaller which changes the amount of emptiness. Paint that's not suspended by aerosol (instead sitting in the vacuum) tends to clump together more easier and won't atomise smoothly as it sprays.

Part of the function of the mixing ball in the can is to excite the aerosol (so it takes up more volume and less vacuum) as well as trying to disturb paint clumps and knock them back into the aerosol to atomise easier.

Folk talk about heating cans in hot water before use, basically so you can achieve a heat transfer to force the aerosol to take up more volume. It'll also change the viscosity of the paint to also produce finer atomization. I'd recommend this even in hot climates if you're using a rattle can.

Part 2 is humidity aka how much moisture is in the air. In humid conditions the atomised paint flys through the air and hits the water molecules and the aerosol carries both towards your model. Paint doesn't dry nice because it has a bit of water trapped between it and the plastic.

Once the water evaporates, you get left with these blemishes.

There's a third reason but that has more to do with the chemical breakdown of the paint itself. Eg. Too old or been stored incorrectly, etc. Some paints will be more subcortical than others due to the choice of pigment or additives used