There's a certain statistical factor in this.
Rotherham when playing well still on average occupy the opponents half (at best) 60% of the time, the last third will be even less than that.
So Hirst with his reduced appearances gets much less "contact time" with the ball.
Keep him on the pitch for longer, his contact time will be longer and his chances of scoring will be better.
Got to start him from kickoff now or why did we bring him in?
Another thought is that PW and the coaching staff observe him all week in training and make a decision on how much game time he's given based upon that.
Most of us don't see what they see