Having sex with a 17 year old is legally no different than having sex with a 10 year old. What changes is that the consent of the 17 year old might be seen as a mitigating circumstance whereas the naiveté of the 10 year old would be seen as an aggravating circumstance.
So, I doubt that he would have been treated the same had he raped a 10 year old. The fact that he raped a 17 year old probably just resulted in less prison time. There's your differential.