The "workplaces" could be just a large group from the same workplace was split into two groups, one for control and one to see the effects of ergonomics on their productivity. If that's the case, the variance in initial productivity would be less than if the two groups were taken from different factories.

Whether or not that's true, it reminds me of the experiments done near Hawthorne, Illinois around the time of the Great Depression to improve worker productivity by changing their environment. The workers output improved almost regardless of environmental changes. The conclusion was workers output improved because someone was paying attention to them. Henry Landsberger analyzed the experiments in the 1950s and coined the term the Hawthorne Effect. (Edit for grammar, spelling, and coffee)