I have a MSI Aero GeForce GTX 1080 Ti 11gb OC with a i7-8700K. I do not rely on Cam for temperatures. I use CUPID HWmonitor for temperatures only because there accurate. I have a different motherboard than you the MSI Z370 Gaming Plus. There should be somewhere that you can turn off the Intel turbo boost for your i7-8700K. So the cpu will stay at a base 3.7mhz. Intel turbo boost on will make it go to 4.7Mhz. I notice no difference what so ever from turning the turbo boost off and having the cpu at 3.7 Mhz max to intel turbo boost on where it allows the cpu to boost to 4.7 Mhz and I have a 32" Samsung Qled 4k at 3440 x1440 Res monitor. OTHER THAN TEMPERATURE. Having intel turbo boost off on the CPU so its max clock speed is 3.7 Mhz. Will greatly improve CPU cooling by 20C minimum wile hard gaming. Far as your GPU being hotter in CAM there is a GPU overclocking area. Where it says power limit it is set at a default of 100%. Set it at 60% all the way down. It does not prevent low performance. The GPU still boosts up to its max. I noticed my 1080 Ti before I found this trick out was running at 100% power causing it to go up to 70C even 84C.
CAM reads the CPU_Fan header on most motherboard. Please verify that you have the CPU Fan connected to the CPU_Fan header on your motherboard. If you have a AIO_Pump, W_Pump on your motherboard you can try those header as well.
If you are using a Kraken, please be sure to have at least one of the radiator fans connected to the 4 pin header on the Kraken fan port. The Kraken pump cable should be connected to the CPU_Fan header on your motherboard as well.
Scroll through the steps on this link. You want steps 7 through 11. Make sure your 2 fans on the radiator are 4 pin PWM. If not, buy 2 radiator fans that are 4 pin PWM so you can connect them correctly. https://www.nzxt.com/manuals/krakenx/X52-X62-115X.html
This is normal and has nothing to do with CAM. But I would not say Cam is completely accurate. I use CUPID HWmonitor to look at my CPU and GPU temperatures and the CPU cores fluctuate in temps and is normal. But I still recommend using CAM just not for accurate temps of the CPU.