Read this link for info about being released with a life sentence.
When people are sentenced to like 200 years, isn't it for a term of "no less than XX years, but no more than 200 years" or whatever.
Or, if they don't have a minimum, and by some really, really odd chance that they live 200 more years, they'll actually be released. Or maybe saying they have to serve 200 years makes sure that they couldn't be released with a life sentence.