How incompetent was the District Attorney?

The DA was naturally relentless in Steven testifying to put Parnell away. The DA knows the sentencing guidelines upon the conviction of ANY crime (5-10 years, 10-20 years, 25 to life). Since the Timmy White trial came first (which Parnell got 7 years), how the hell could he not have known Parnell wouldn’t get any more than the 20 months he received?! He should have known immediately after being sentenced in the Timmy White case that a 20-month sentence was the maximum he would get.