Java: ExecutorService less efficient than manual Thread executions?

I've got a multi-threaded application. When using Thread.start() to manually start threads every concurrent thread uses exactly 25% CPU (or exactly one core - this is on a quad core machine). So if I run two threads CPU usage is exactly 50%.

When using ExecutorService to run threads however, there seems to be one "ghost" thread consuming CPU resources! One Thread uses 50% instead of 25%, two thread use 75%, etc.

Could this be some kind of windows task manager artefact?

Excutor service code is

ExecutorService executor = Executors.newFixedThreadPool(threadAmount);

for (int i = 1; i < 50; i++) {
    Runnable worker = new ActualThread(i);
    executor.execute(worker);
}
executor.shutdown();
while (!executor.isTerminated()) {

}
System.out.println("Finished all threads");

and Thread.start() code is:

ActualThread one= new ActualThread(2,3);
ActualThread two= new ActualThread(3,4);
...

Thread threadOne = new Thread(one);
Thread threadTtwo = new Thread(two);
...

threadOne.start();
threadTwo.start();
...