The USGA GHIN system, and any calculators using that forumla, only use the lowest 10 of the last 20 posted scores. The formula then reduces the average of those 10 differentials by another 4%, so your index will always be lower than your scoring average. Your handicap is supposed to show your scoring potential, and you should only match or better your handicap about 25% of the time. If you beat your handicap more than that, then it will drop lower to bring it back into that average.