Question about Sim. Misc. tabulations

While attempting to figure out player years, I sometimes track the Sim Misc numbers from game-to-game. As of yesterday, my relief pitcher, Frank Williams, was 1 for 6 on ballpark singles and his rolls were 81-hitter/71-pitcher. In today's games, Frank Williams appeared once, facing only one batter (Carney Lansford) and giving up a single. Under the Sim Misc tab, his numbers stayed identical except his Inherited runners ticked up from 30 to 31 and his GB changed from 67 to 68. What does this mean? If it was a gb(x) on the pitcher's card shouldn't that also appear as pitcher roll?

I would assume SOM counts some SINGLE results (or maybe just SINGLE*?) as groundballs. I don't know for sure, however.