in 1986 McGee drove in 16% of the runners on base when he batted
in 1988 McGee drove in 14% of the runners on base when he batted
in 1987 McGee drove in 17% of the runners on base when he batted
So, a slight improvement in the percent of runners he was able to drive in.
BUT
in 1986 McGee had 291 runners on base. If he had driven in 17% (instead of 16%) he would have increased his RBI by about 3 RBI giving him 51 RBI
in 1988 McGee had 369 runners on base. If he had driven in 17% (instead of 14%) he would have increased his RBI by about 11 RBI giving him 61 RBI
in 1987 McGee had 544 runners on base.
That's why he had 100 RBI in 1987