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 σ_{M1-M2} is very large and so the 3 points is not found to be significant. In the other study, the σ_{M1-M2} is very small and so the 3 points is found to be significant. What might have caused this big difference in the σ_{M1-M2} for the two studies?