Question CPU under volting

[SWIZ]

Hello, I have a i7-8700k, which I did overclock and it runs at 4.7 MHz rn at a 1.300V.(Bios settings)
I used an Intel Extreme Tuning Utility later, and tested it at 4.8 with voltage offset set to -0.115V. The system was stable and I run it for about a month. I just found some weird thing that the overlock from IETU not always applies when booting windows. So seeing that the system is stable I decided to put those values into the bios settings directly. I did turn off the “apply overclock on startup” setting in IETU and change my bios settings to 4.8MHz and 1.300-0.115=1.185V. So now the system doesn’t even boot and crushes on startup(
When I returned to 1.300V everything is fine again…
So can someone tell what is the mystery here? Perhaps I did something wrong with the voltages and I wanna hear some advice

 

[SWIZ]

Running it under load shows me following values in Hardware Monitor:
CPU Vcore= 1.152 with max at 1.272V;
VID0 to VID5(core voltages I presume)= 1.360-1.380V average with max at 1.469
So how can that be if my voltage set to 1.280V rn in the bios(tried 1.250, but it crashes almost immediately)? The only thing I can think of is the Boost voltage or sth. Can someone explain it to me in more details?