Laws in each jurisdiction differ, but in some jurisdiction certain crimes carry specific sentences. These sentences usually have a minimum and maximum time they can be set to, and someone convicted with multiple charges has to be sentenced for each and every one of them. Also depending on the jurisdiction the judge may have the ability to add more years on top of that on a case by case basis. This is important because cases can be appealed even after sentencing or even reversed if new evidence comes to light. Thus if someone was charged with multiple crimes, but only charged with the one with the highest penalty, if they could successfully appeal against it they'd walk free, whereas when they have multiple sentences for multiple crimes they have to fight each individually, meaning they're much more likely to serve time, even if they can successfully fight one of the charges (which doesn't necessarily mean they were innocent, just that they could argue there wasn't enough evidence against them)

Then we have life imprisonment, which as a sentence can mean different things in different jurisdictions. In some countries it's more or less what it says, imprisonment for the rest of one's natural life unless pardoned or paroled. In other countries however life imprisonment may actually just mean a fixed sentence (of anything from 15 to 45 years) after which they may still be let free. In some jurisdictions sentences on multiple charges can stack even to an unnatural length (being sentenced to 150 years for example), while in others regardless of the charges against someone there may be a maximum year limit to which someone can be sentenced, even if the sentences for each individual crime committed add up to more than that.

So in the case where someone has committed a heinous crime, but the life sentence seems small compared to the act, more years may be added. Basically the important thing to realise here is that 1) Laws may vary a lot from one jurisdiction to another but 2) Legal terms always have a legal definition in their jurisdiction. These terms may often be very confusing since their meaning in common language may mean or imply something different than what they legally are. This is the reason why it's almost always a bad idea to defend yourself in court. The job of a lawyer is more or less knowing these terms and how the system works and help you, because without studying a legal system it's impossible to understand.