Stability and consistancy are not words that really get me excited with a unit that has been pretty bad for three years.
I understand the point you are trying to make, but statistics need to be assessed within the context that they were generated.
First, one has to make a realistic assessment of the talent within the program.
PR took over a program on a 10 game losing streak. There were two coaching changes in a two year period. With each staff turnover, recruiting is going to suffer. Thus, we entered the PR tenure with a lack of talent, and the multiple staff changes made it impossible to fix this problem in the short term.
Second, each coaching staff was running a different offense and thus recruiting players that fit their individual system. As a result of three different systems in a span of three years. The current staff has upper classmen that were recruited to fit a different offensive philosophy.
The very lack of coaching stability and consistency and it's impact on recruiting is largely responsible the offense being behind the rest of the conference. It will take five years for PR to fill the system with players that fit his vision for the offense.
Missouri is a great example of how long this process takes. Drinkel was on the verge of being fired around year five or six when things finally started to click.
Futhermore, I would argue that ISU's schedule has been increasingly more difficult each year of PR's tenure. Thus, gradual improvement may be masked by a tougher schedule.