The reason why higher average bowlers are given the slight (10% or 20% advantage) is due to the fact that the most you can throw in one game is 300.
Although it is an extreme example, it will demonstrate my point. Consider a 220 average bowler and a 60 average bowler bowling in a tournament where the handicap is 100% of the difference between your average and 220.
If the 60 average bowler bumps a few up against his leg, falls down, slips out of the ball and gets a triple and a couple of lucky spares to squeak out a 141, his score is 141 + 160handicap = 301. The 220 average bowler simply can not win, even if he throws a 300 (300 + 0 handicap = 300). Is that fair to the 220 average bowler who just threw a perfect game? Which is more difficult? A 60 average bowler throwing a 141 or a 220 average bowler throwing a 300? If the handicap system is 90% instead of 100%, now the 60 average bowler gets 141 + 144 = 285 and the 220 average bowler still gets 300. (As a side note, a 240 average bowler gets the same thing as the 220 average bowler in both scenarios). In essense, the 220 average bowler is given a true 16 pin handicap to make up for the all-too-common scenario of the 60 average bowler getting "lucky". Remember, lower averaged bowlers are less consistent, so their scores overall will be spread over a greater range of scores.
The 10% or 20% advantage given to the higher averaged bowler is a sort of recompense for the fact that they are limited in how many pins above average they can be.
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