Lets say two groups are compared. Subjects are randomly assigned to each group, then a treatment is given to half while a placebo is given to the other half. All aspects of the experiment (order of treatment, etc) are also randomized. The treatment group gets a much higher "score" than the placebo group on average, but not all in the treatment group score higher than all in the placebo group. When this occurs it would be usual to attribute the average difference to the treatment.
I would say two things against this:
1) The "weak causality" argument. The results are conditional on the exact environment under which data was collected. The treatment may not increase the scores if some seemingly minor aspect of the experiment is changed. In other words, the effect of the treatment is only "unmasked" if some other (unknown) criteria is/are met. The exact conditions of the experiment will never be repeated again.
2) The "randomization guarantees nothing argument". Randomization does not guarantee that the two groups are balanced on all important factors at baseline. It only makes it unlikely there is severe unbalance.