In my experience most teachers hate giving over 100 on anything, at least at my school. Most of the time you will find "extra credit", if given at all, is usually worth less than the amount of work that is called for.
"I am choosing to work that problem because I beleive it is worth 8 points. "
You recieved a 98.72 on the test. With the credit, you recieved a 106.72. Looks like eight points to me! If he meant 8 points relative to your grading scale, I think he would have said so.
All in all, does it really matter? If I were a professor and someone bothered me about something like this, I would probably grade the next test a bit more harshly. Some of my professors have said they'll take points off for having to look at little errors.