I was testing a bit with different cores and layers of carbon under the core and one of my friends asked me what causes the core to print through to the carbon. And I honestly don't know the cause. This was Lantor Soric core in the example below.

I know and expect that it happens when using only a single layer... but the why is a mystery to me. Is it just that the core compresses more than the carbon under vacuum and moves the weave a little bit?
Pic for an example:

Looks like you have a precise core which is X shaped.  What happens on the edge of the foam creates a void as the fibres lay over the edge rather than conform to the 90 degree edge. Then during the infusion, the fibres lift on the surface creating that effect. Basically its the same as bridging you might have on a corner as the core is creating a corner.  You can minimise it by tapering the core edges (not so easy on Soric but easy on more rigid cores to sand it) or use carbon tape or offcuts cut in strips around the edge of the foam to taper it down so it can't bridge like that. 

That won't stop the slight hexagon pattern you get which is down to the tiny amount of shrinkage during the cure.  Thicker laminate reduces that effect.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant