He was probably released from prison in Greece after only one year, because Greece is financially on its knees and cynically whoever made that crazy decision knew that he'd move on to northern Europe as soon as he could.
He ended up in Germany because Angela Merkel and her naive policy of opening Germany's borders and welcoming all-comers without vetting them struck his violent, criminal mind as an excellent opportunity to go somewhere new where he was unknown.
I'm willing to bet he hasn't been 17 many a long year, but he knows, as well as any mug who's entered Europe and cried 'Asylum' as soon as anyone in authority approaches, that claiming himself to be a minor gives him access to special treatment and privileges. The EU (and in particular, Angela Merkel), created this situation when they failed to put a proper vetting system in place before they allowed hundreds of thousands of people to flood, unchecked across Europe in 2015. It should surprise no-one that in that number were criminals, terrorists and other undesirables. And now, safe in their new, western countries, where asylum systems are so overwhelmed that their paperwork is unlikely to be processed any time soon, they're free to grope, rape, murder and maim at will.