The stimulus already controlled for different amounts of typing: “among those who do similar amounts of typing...”

The stimulus told us carpal tunnel syndrome is a nerve disorder. If feeling lack of control can make one more susceptible to nerve disorders, this could account for why those reporting the least control get carpal tunnel syndrome (a nerve disorder) at a higher rate than those reporting the most control.

This doesn’t differentiate workers reporting the least control from those reporting the most. We have no reason to think workers reporting the least control are more likely to use the keyboards described in this answer.

The stimulus already controls for typing amount. Whether workers rarely use keyboards or use keyboards a lot, among those who do similar amounts of typing, people reporting the least control get carpal tunnel syndrome at a higher rate. Why? This answer doesn’t provide a theory.

This makes the discrepancy more difficult to explain. If those who have the most control perform repetitive motions more than those who have the last control, we’d expect those who report feeling the most control to get carpal tunnel syndrome at a higher rate.