It's not, and they don't. For non-heinous crimes (i.e., crimes that aren't rape or murder) prosecutors threaten the maximum sentence for the "worst" crime charged, and only that number, because any competent defense attorney would immediately let their client know that sentences wouldn't be served consecutively. This is true in most, but not all states as well. (For heinous crimes, the consecutive sentences don't matter; the defendant has already received a life sentence and the additional years are meaningless.)

This is also a legal ethics issue, since a prosecutor who fraudulently claims a higher maximum penalty during plea bargaining can be suspended or disbarred. Cops can lie to you--prosecutor's can't. They have sworn an oath (to the bar) to be honest in their legal representations. Prosecutors have been sanctioned, suspended, and even disbarred for being dishonest about sentencing because such dishonesty indicates moral turpitude that likely extends into other facets of their legal work.

It's the media that throws around the 100-year numbers by adding together all of the potential individual sentences. It turns out that journalists aren't very good at understanding technology, science, or the law.