Let's assume your CPU is a dual core, with Hyper-threading, giving it 4 virtual cores [I know this isn't true, but bear with me.]
If you run a task that is single-threaded - simply, it can only do things serially, one part must finish before the next bit starts - then the maximum amount of CPU it can use is one core at a time. This is 25% 'busy' when looking at your total CPU time.
Now, the scheduler in your OS is a bit smarter than that, so it can shift tasks between cores to stop one getting all the fun & share the workload out a bit. It also isn't only doing that one job, your machine is running hundreds of tiny background tasks all the time, even if you personally are doing no work.
Long & short is that if a task is not a real CPU hog, demanding more & more parallel threads/tasks, it will only get one turn on the roundabout, it isn't going to get all the seats at once. This will limit how much work it can do at any given time & how 'busy' it will make the CPU. If it took the whole CPU, then everything else would grind to a halt.
The only kinds of task that get to sit in every available seat are things like video encoding & application compiling. These will make your machine sound like it's about to take off, as every single spare resource gets used up & the fans ramp up like a wind turbine. For more normal tasks, it's nice to be able to get some elbow room for other things you are busy with, and also keep your machine cooler & quieter when it really doesn't need the exercise.
Change graph to -> Logical processors
. This should immediately show that one of the cores is doing the heavy lifting while the rest isn't doing that much at all. Some applications are optimized for using multiple cores (you'll often see 2-4 active cores with those), others are not.