I'm not sure I understand the question correctly. It's not assumed that the criminal would have received the minimun sentence. Actually most cases of reduction I have heard of were in the upper range. 10 to 12 years that get lowered 2 or 3 years.

The court compares the original conviction with how they would convict with the new law, all facts remaining the same. If this results in a lower sentence then they have to reduce it.