Should life mean life ? - debate

Here in the uk if you are served a life sentance you will get twenty five yrs , its very rare that you will serve that full sentance, its more likely to be halfed or less with good behaviour. In the usa, life means life, mostly served without the right to parole, even if you get sentanced to life with the possibilty of parole it takes a long long time for them to even consider it. Why is it so different in each country and what in your opinion is the correct answer, should life mean life ? or is the sentance just a mockery to those left behind? x