The advantage of the GSM system is the paper weights are consistent regardless of the paper type. The Lb system has to do with the weight of the blank they are cutting it from. For example 500 sheets of 22" x 17" paper weighs 20lb, cut in 4 sections gives 4 reams of 81/2 x 11 20lb paper. 500 sheets of card stock weighs much more.
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