The tests to measure violence mean nothing by themselves. It is by comparing the results that you can draw conclusions. In this case, they compared people who played violent video games with people who played non-violent games.
The study shows a difference in both cases. However, if they didn't test with people who did not play either violent or non-violent games, they lack a control group and they cannot say if violent games make you more violent or if non-violent games make you less violent.
The study shows a difference in both cases. However, if they didn't test with people who did not play either violent or non-violent games, they lack a control group and they cannot say if violent games make you more violent or if non-violent games make you less violent.