Considering the length of prison sentences of murderers compared to child rapists worldwide I'd say it's really common, if not standard, for murder to be considered much more serious except in the most brutal and violent cases of sexual rape and sexual abuse against children (or people in general).
There's also a cumulative element, so that multiple offenses add up. I'm just guessing, but I'd predict that multiple offenses (and hence, added time) is more common in child abusers than murderers.
Whether or not adding up is merited is another good question. In this case, although the final number seems a bit silly, the crime was over-the-top terrible. But the same mechanism that would get someone a life sentence for multiple sales of stolen property? Harder to justify.
Here's one from last month:
A Marion County jury convicted a Hannibal man on five drug-related felony charges on Monday, July 27, which could lead to a total sentence of 120 years in prison.
During a one-day trial at the Hannibal courthouse before Judge Rachel Bringer Shepherd, Marion County jury members found Jimmy D. Atteberry, of Hannibal, guilty of five drug-related charges: one count of possession of a controlled substance with the intent to distribute, two counts of distribution of a controlled substance and two counts of possession of a controlled substance. The drugs included methadone, amphetamines and marijuana.
(http://www.hannibal.net/article/20150730/NEWS/150739877)