First of all, if by marbling you mean spreading chocolate on a slab, then your chocolate may be tempered. If you heated the chocolate properly and slabbed a good portion of it then when you add the slabbed chocolate back in, it will temper all of the chocolate. Your "problem" may be that the chocolate was actually tempered.
I'm also a little confused about how you are judging the interior to be out of temper. Other than the "snappiness", the quality of the interior chocolate is almost never used to judge temper. If you could provide a picture of what the inside and outside looks like that would be very helpful.
Aside from that, there are a few possibilities:
- How long did you wait before judging the temper of the outside? Chocolate may not show signs of streaking for up to 24 hours.
- Polycarbonate molds when used and, more importantly, washed correctly will build up a layer of cocoa butter. This layer of cocoa butter may help encourage the chocolate to form the correct crystals (temper).