What does a 10 year cap mean in criminal law

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My son was told that there would be a 10 year cap, and most likely 8 years, on his sentence. Worst case senario, 5-10 years. He waived his preliminary hearing because he was told that if he went to trial he could get as much as 10-20 years. He was sentenced to 8-16 years. When he asked about the 10 year cap, he was told that was on the minimum. Now, feeling deceived, he wants to file a PCRA. In your opinion, is this grounds? If so, would he have to go back to square one in the process? Is there some way to have his sentence adjusted without starting the process over? His lawyer is telling him that the sentence was legal because it was below the mandatory minimum of 10 years. We are in the process of requesting the preliminary hearing transcripts because he is sure that his lawyer noted the 10 year cap to the judge.