The average "life" sentance in Germany served is 20 years.
20% serve until a natural death if they are deemed a danger to society.
He got out in 16 years, 2 less than he was sentanced for at a MINIMUM.
Edit: Ok since he was arrested in 1987 they most likely gave him time served as well, which puts him just over the 18 year minimum sentance. He still of course got out a year ealier than the 'average' life sentance and apparently people who are caught with liquid explosives, hijack planes, are known members of a terrorist organization, and kill hostages are not a 'threat to society' according to Germany.