The improved accuracy is all due to the increased guess variance between groups, right? If so, it isn't an unintuitive result. I feel as if the authors cheated

The improved accuracy is all due to the increased guess variance between groups, right? If so, it isn't an unintuitive result. I feel as if the authors cheated a little bit in the abstract when they wrote that their findings go against the "prevailing view". Deliberation is expected to reduce the guess variance within groups, but not between them. Great paper, anyway. But it made me wonder: how large the groups can be before they start reducing the accuracy of the aggregate guess?