Completely normal, old games aren't very good at using multiple cores.
Hello... Your audio needs its own cores too... and Skype... etc... would it be smart to scatter all the information evenly between them? Maybe? But the programmer's of the game didn't see that way. Did you know you can tell which cores to do what with a click of a button? we can make the second core go to 90% only... Would you feel better? Or next week run #3 at 90% for a while.... And then the week after that... B )