The Core0 C6 counter is really zero, so it looks like that core was never in C6 state (during your run-time).
When a counter is 0, it's not possible to know whether it's present or just didn't count anything yet, so it's not shown in HWiNFO. If you Core0 enters C6 state, the counter will start counting and it will appear in HWiNFO.
however my guess still seems to be right. Luckily i have 2 systens (same hardware, different bios) to test. And my other system is behaving in the same way (not showing core0) as soon as i disable c6 states there. So i guess for some wierd reason c6 states are disables on my beta-bios system.