The reason is that your handicap is not the average of your scores.
For example, if you are a 10 handicap it does not mean that you average 74 (par 64 + 10 handicap) around the Roaring River golf course. More than likely you average around 78-80.
Handicaps are calculated using the best 10 scores from you last 20 scores. So your terrible rounds are not even included in your handicap calculation. This also means that a really good round will bring your handicap down, whereas a really bad round will probably not change your handicap.
STEP 1 :
For each individual 18 hole we find the handicap differential. This is done with the following calculation :
Your score - Course Rating x 113/116
A score of 74 at Roaring River course gives you this handicap differential :
74 - 66 x 113/116 = 7.79
113 is the average course Slope Rating.
Roaring River Course Rating is 66.
Roaring River Course Slope Rating is 116
Now we have the differential of 7.79. You calculate this differential for the best 10 scores from your last 20 rounds.
STEP 2 :
Average the handicap differential for these best 10 scores from your last 20 rounds. Add them up and divide by 10.
Mulitply this number by 0.96 and you now have your handicap Index. This handicap Index can be used to calculate your handicap on any golf course you play on in the world.
Example. If you Handicap Index was 12.7 then we use the following calculation to find your handicap for Roaring River Golf Course :
12.7 x 116/113 = 13.03 (116 is Roaring River Slope Rating).
Therefore your handicap at Roaring River would be 13.
Fortunately we do not have to do these calculations for every player. We enter the scores into the computer and it does all the calculation and then gives us the handicaps.
It is also a little bit more complicated than this because there is something called ESC (Equitable Stroke Control). This factors in "Blow up holes". If you want to read more about ESC go here :
All the information on Calculating Handicaps can be read here : http://golf.about.com/cs/handicapping/a/howcalculated.htm