Number of available cores
Summary: the more cores you use the more battery you consume so CPU hotplugging turns off the other cores, and the others do the work which makes them at 100% CPU usage, which also give the best performance and the most balanced one.

It's CPU hotplug It turns on and off the cores when needed or not.

It's running 100% CPU usage because it's like this. Imagine, there's a 4 car, and let's assume the drivers are the cores. You have four lanes and 4 cars of course. Each one of them will take 100 pizzas. You can let one  core to do all the work and that's what hot plugging do. Knowing that a car with 50 pizzas consumes a lot less fuel than a car with 100 pizzas you can now see how we can have the same performance and save on fuel: load two cars with 50 pizzas and let them accelerate as quickly as possible to the destination (performance governor). They will get there as quickly as one car with 100 pizzas (same performance), but burn less fuel (battery saving).

If you want to make your phone run at 8 cores then you need to install a custom kernel, use an other CPU governor, use a custom CPU hotplugging, or without installing a custom kernel use an interactive governor, then tweak its parameters.
You can search on XDA for Advance Interactive governor tweaks.