It is likely that your 7820x system has some other processes that are using the CPU cycles. Perfectly normal if you are using the computer for daily tasks. But if it keeps up, you can sort by CPU usage and see what other tasks are using the rest of your CPU.

I don't know enough details to give a definitive position, and I don't have the time to do the reading, but I do know that, on some chips, you can't always run two hyperthreads on each CPU because certain advanced instructions use components that are either shared between two CPUs, or that can't be scheduled on both hyperthreads in the same CPU. If you're running a project that uses these advanced instructions, and your chip has one of these "cheap" layouts, then you will run into scheduling issues.

I'm not being disparaging. It's just a choice that the manufacturers made to hit certain price points on their chips, and it's not always obvious to the poor consumer -- especially ones like us who hit systems hard trying to run all possible threads flat out. You might read about it in the spec sheets if you're lucky, but reviewers usually spot things like that. It's always worth doing some deep reading before you settle on a chip for running WCG 24/7.