Because it seems like High, Ultra High and Extreme on ASUS Z77 boards always results in vRise. Plus, higher LLC levels results in higher VRM temps.
Besides, the voltage set in the UEFI BIOS isn't the actual voltage that's going through the CPU.
So, the reason why you get instability and BSODs with the LLC set to Medium is that the full-load voltage isn't high enough. I mean, it's not because it's a bad LLC setting. All that really matters is what the full-load voltage is, however using lower LLC levels also results in lower VRM temps.
So if I am understanding you correctly using medium setting and higher bios vcore to make stable actually will reduce temps? Id like to goo for 4.8 myself but it needed 1.4v min