In two separate studies, the actual difference between the means of a
treated group and an untreated group is 3 points. However, in one study,
the σM1-M2 is very large and so the 3 points is not found to be significant. In the other study, the σM1-M2 is very small and so the 3 points is found to be significant. What might have caused this big difference in the σM1-M2 for the two studies?