You will occasionally find a mould that produces pieces with surface pitting to some degree or another. 

This is caused by airbubbles getting trapped on the surface of the mould.

There are all manner of interesting technical reasons for this, mostly involving surface tension.

The worst culprits are moulds where there are large flat smooth surfaces. These create conditions where air has difficulty finding an escape route, so it just stays put and creates a pit. One way around this is to slightly roughen the surface of the mould. This creates fissures/features to both encourage the air to escape and break the surface tension of the bubbles to allow the metal in & vice versa.

Below are a series of photos of castings from a problem mould, where the surface of the mould had been polished to a near glass finish. I roughed it a tiny bit with 600 grit carborundum paper, and got a better result, and made a second pass with the emery paper and get a splendid finish. 

no detail and a polished surface = Pitted casting

Lots of detail and a non polished surface = Clean & Shiny casting

Polished

One pass with Emery Paper,

second Pass, 

Magic!

(all I had to do then was sort out the edges so it cast all the way to the rim. Which I did.)