I think the researchers are seeing the effect of mindfulness in action. Changing the conditions forces the body&#x2F;mind to not go on auto-pilot and focus on the minutiae of the task. Consequently they improve through this deliberate practice. The changing conditions is forcing people to really think and focus on the task at hand.
I would think it is also important to repeat things with same conditions long enough for the muscle memory&#x2F; heuristics to kick in. If you change conditions rapidly there will surely be no learning involved.