Justice?

I have to admit my complete lack of understanding of the Scottish legal system.

This week the trial of a convicted murderer and rapist was thrown out due to lack of evidence. Forensic evidence linked the accused, both victims and his vehicle at the time. The key element in the dismissal of this case appears to be the failure of the prosecution to present any evidence showing a forensic link between the accused and the ligatures used to murder the two young women.

A couple of years ago a 16 year old boy was found guilty of murdering his girlfriend when he was just 14 in a "savage knife attack". There was no forensic evidence linking the boy to the crime, nor any witnesses placing him at the scene.

I just don't get how this works...