HP&Torque differences in years for 3.1 v6

I have a 91 z24. The book shows that the 91 year 3.1 has a good bit lest power than the 92-94 years. What gives? Is it the ECM or cams or something else?

There should be a 5 ft-lb difference in power between 92 and 93. It was a cam difference, if memory serves.