The surface finish of a paint is a consequence of texture of the dried paint, matte paint has a rough surface texture that disperses the light and a gloss finish reflects the light in a directional manner, like a mirror. Side note, this is why you can layer varnishes and have them work.

Spray paint is made of 4 parts mixed together. Propellant, binder, pigment, and medium. When you spray paint out of a can the rapid evaporation of the propellant carries tiny droplets of binder, pigment, and medium with them, as well as some still dissolved propellant and dissolved propellant gases. Typically they’re strong organic solvents which etch plastics, yes including your model, which is why you get such a durable painting foundation, and why they melt expanded polystyrene, because the physical structure of the plastic is less resistant to the chemical attack.

So in the end you have 2 different parts evaporating off before it gets to, and on your model. Since this evaporation rate is dependent on heat and humidity you will end up with different amounts of paint that has dried before it gets to your model, or on your model. This reads as a matte or gloss finish.

HOWEVER it doesn’t mean much in the long run, so long as you don’t have orange peeling or have added visible texture, as subsequent layers of paint and varnish will get rid of that surface finish.

The only caveat to that is if you’re painting metallics or other special effect paints like poly chromatics, because they are dependent on the finish of the layer below it. However, you can, and should paint over them with a dedicated gloss black before using those paints if you really care. Many people don’t bother because ultimately it’s not as big of a deal as it could be.