system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why are prison sentences stacked upon each other?

For example you're reading about someone convicted of a crime and they get...2 consecutive life sentences. 

Or where one guy gets 300 years and another gets 500 years?

I'm not sure of the additional years are meant as a punctuation to a sentence that reflects the crime, or isbthe judge/system trying to cover their bases in case of life extension becoming a thing? (And even if life extension is discovered, that person would be serving a sentence, and probably not eligible or rather able to get/receive whatever the extension would involve)

So, anyone wanna break this down for me?

*Not sure if my flair on this relates to economics, so putting it on other for now*
gpt
You can serve time “concurrently” or “consecutively” for multiple convictions. Someone serving concurrent 2-year sentences for burglary and weapons possession should in theory be out of jail in two years.  Someone serving a consecutive sentence for the same thing would in theory be out in four years. 

Now what people don’t know is that in some areas you get “credit” for every hour you don’t cause trouble in jail. So if you don’t cause trouble for an hour, you get credit for an additional hour of jail time.  So in the previous concurrent example the convict would be out in just one year and in the consecutive example he would be out in two years. 

How the sentences are handed down is often dictated by state sentencing guidelines. The judge has a series of checks that determine the length of sentence and how it is to be served. If there is a prior criminal past that pushes the sentencing one way and if there is no history then it pushes it the other way. Things like employment, family in the community, job, home ownership, etc. can also play a role in sentencing.  

For most serious crimes parole or early release is also possible if a percentage of the sentence has been served (I believe like 75-85%).   So after 8.5 years of a 10-year sentence someone could be eligible for early release.  In particularly heinous crimes, or frequent convictions, the sentences are designed to prevent early release by assigning more years to a sentence and making it a consecutive sentence rather than concurrent.  

So a child rapist who accidentally kills their victim might only have gotten convicted for second degree murder, or maybe even just manslaughter.  Those would have significantly fewer years of potential punishment than first degree murder.  So for the rape the guy might only get 5-years and for manslaughter he’d get maybe 10-years. All total he could serve as little as just 5-years (remember he gets an hour of credit for every hour he serves with no trouble) and if the 5 and 10 year sentences are served concurrently then he would be out in no time. 

By making the sentences consecutive he would serve as little as 7.5 years and as many as 15. 

But a lot of it has to do with prior criminal history.