Why are rates of erosion of rocks (specifically marbles) different for each time?

We did a marble lab today on weathering and abrasion, and one of the question is,"Give two reasons why the m lost was different for each group.". We had groups, and we put the marbles in a shaker bottle, and then we shook them. We then measured the m and the m was decreasing because of abrasion. But why? I know one reason. Because the shaker of the bottle might shook the bottle at a different rate. But what's the second one? I can't think of one!