That's because whatever you're running at that time is made for single threaded use, meaning it wont take advantage of multiple cores. If you're running multiple applications/programs you can set which cores they work on to even the workload across your CPU.
To do this in Windows 10:
1. Open Task Manager by right clicking Windows Taskbar.
2. Select 'Details' tab.
3. Right click the program and select 'Set Affinity'
4. Select the Cores/logical cores you want to run the program on.