Without knowing more about the sprue/gate/runner configuration, I would guess that you have a sand inclusion.
I found a good summary about casting defects not too long ago.
I will dig that out and post it.
As I recall from the summary, round-edged defects are often gas defects.
Jagged-edged defects are often caused by sand failures.
You can also have turbulence defects caused by excessive metal velocity, where you can entrain sand, air, and slag into the moving molten metal.
I use a hybrid John Campbell / Bob Puhakka sprue/runner/gate/riser configuration, with my own modifications, and I have not been having casting defects.
John Campbell published 10 Rules to Good Castings, and I follow those rules pretty closely.
I can write up a summary of how I do it.
Much of it is about controlling metal velocity, avoiding air aspiration during the pour, avoiding abrupt transitions in the sprue/runner system, and using risers where appropriate to avoid hot tears.
Perhaps posting some definitions of casting defects would be in order.