It's not uncommon to want to clear all the performance data gathered during the session so far, and then continue monitoring without restarting the session.
Resetting the statistics does exactly that.
Or use the Reset All Statistics icons on the Session toolbar.
The way this works is that a message gets sent to the stub monitoring the target program.
All threads in the target program get suspended whilst the statistics are reset.
Once reset, all the threads are resumed and new values for the statistics are calculated.
There may be a slight pause before the statistics are reset.