I received my kit a few days ago but have not had time to put it together yet. I have my piston here in front of me.

"Skirt" normally refers to the sides of a piston, the part that contacts the cylinder wall (not really, as there is oil between them). The skirt of my piston is machined nicely.

What I think you are referring to is the top of the piston which seems to be un-machined except for a 3/4" circle in the middle. I too wondered about this but decided that it really does not matter. Carbon will quickly collect and fill in the roughness.

I think the top of the pistons are left this way intentionally. Note that the machined area is actually higher (raised) than the rough area. The chinese probably think that the roughness will cause better mixing of air and fuel, or better distribution in the combustion chamber.

Flat top pistons are pretty common. I have not seen a domed piston in a model engine, gas or glow.