It may be that people who drink diet soda consider it a treat, so they doesn't indulge in other fattening treats. The people who drink plain water might succumb

It may be that people who drink diet soda consider it a treat, so they doesn't indulge in other fattening treats. The people who drink plain water might succumb more easily to temptation. But we don't know it this is the case, because the people who did the study probably just run to publish it the minute they got the results.