Suppose I toss a brick through the window of a government office overnight when no one is inside, causing $101 worth of damage. I could be charged with felony destruction of government property, a crime with the statute says has a maximum sentence of 10 years and a $250k find.
Am I facing 10 years in prison? No! That's because the way sentencing works under the Federal Sentencing Guidelines is that the sentence range is looked up in a table whose rows range from short sentences up to the maximum (10 years in this case). The biggest factor in that lookup is how much monetary damage I caused. At $101 I'm going go get a row with a sentencing range that is something like up to a year or so.
To push it up to where my sentencing row actually includes 10 years I'd have to have done a lot more damage, such as blown up the building.
(There are other factors that also go into picking the row, such as if I have prior convictions for similar crimes).
When I'm indicted, though, the DOJ press release will tout that I've been charged with a crime with up to 10 years and $250k fines as penalties. They'd say the same thing when indicting someone who blew up the whole building. Their press releases make no attempt to convey the sentence the person is actually facing based on the facts the DOJ is alleging about that person's alleged crime.
In the case of someone charged with multiple crimes there is another factor that leads to inflated numbers. The same underlying acts can often support more than one charge. Similar crimes are grouped together for sentencing under the Federal Sentencing Guidelines, and if you are charged with and convicted of more than one crime from the same group based on the same underlying acts, you are only sentenced for one of them.
Unfortunately, this grouping is often ignored in press releases and reporting about potential sentencing.
In most cases, those multi-decade numbers you see people talk are due to one or both of the aforementioned issues: (1) not taking into account what the person is actually alleged to have done, and (2) ignoring grouping if the person is charged with more than one thing.
Here is a detailed analysis of Swartz's possible sentencing [1]. The earlier article by the same author looking at what he was charged with is also interesting [2].
[1] http://volokh.com/2013/01/16/the-criminal-charges-against-aa...
[2] http://volokh.com/2013/01/14/aaron-swartz-charges/