Pendulum_Demo Standard Deviation Wrong

[PeteLustig]

Describe the bug
Sometimes the standard deviation printed out by the pendulum_demo is wrong.
When the Min(13459 ns), Max(83599 ns) and Mean(24672.386000 ns)
values are like in the brackets, how can the standard deviation be 135818791.307981?
It just happens sometimes, sometimes the value is as expected around 1000.0.

To Reproduce
Run the pendulum demo multiple times with all the necessary priorities:
ubuntu@ubuntu-rt-1:~$ pendulum_demo -i 1000

Expected behavior
That the standard deviation matches the other values.

Screenshots
ubuntu@ubuntu-rt-1:~$ pendulum_demo -i 1000
Initial major pagefaults: 0
Initial minor pagefaults: 2124190
No results filename given, not writing results
rttest statistics:

• Minor pagefaults: 0
• Major pagefaults: 0
  Latency (time after deadline was missed):
  • Min: 13459 ns
  • Max: 83599 ns
  • Mean: 24672.386000 ns
  • Standard deviation: 135818791.307981
    PendulumMotor received 498 messages
    PendulumController received 499 messages

Desktop (please complete the following information):

• OS: Linux Ubuntu 20.04 with PREEMPT-RT Kernel
• Version [e.g. 22]