It comes from a particular concept of Presbyterianism (and Calvinism as a whole), in that one must not utter a false statement in Gods name (to do so is bearing false witness).
This has a particular effect on court proceedings, as the Presbyterian Kirk of Scotland, is the official (but not established) church of Scotland. Courts and juries are (or at least were... not sure if it's still done) sworn to deliver their verdicts in Gods name, and trials are conducted under and by the grace of God.
Because of this, in Scotland, as in no other nation on earth, there are in fact three possibly final verdicts for a completed case:
Not Guilty - The accused did not commit the crime, and is not legally responsible for it.
Guilty - The accused committed the crime and is legally responsible for it.
Not Proven - The accused likely committed the crime, but there is insufficient evidence to prove this beyond reasonable doubt.
Note: In fact, although Calvinism as a whole is Swiss in origin, the Presbyterian movement and church (technically churches, as they are individual and separate churches in each nation, under the auspices of the general assembly of the Presbyterian Polity), was founded in Scotland. I can't find if "not proven" was ever an element of Swiss law, but I know that it is not now.The verdict of "not proven" is still an acquittal... it's just an honest one "We think you did it, but we can't prove it, so we're not sending you to jail... but we still think you did it".
The only reason this third verdict exists, is because of this precept that finding someone not guilty, when you know them to be guilty but cannot prove it, is an offense against God. It's uttering a false statement in Gods name.
I really think it's a useful concept though, even without the idea of God being involved.
Oh and the right of juries to bring in a "not guilty" verdict for a charge which is proven beyond a reasonable doubt, but for which the jury believes the accused holds no guilt, or that there was no true crime (malum prohibitum vs. malum in se; or crimes where mens rea and negligence were absent); is an explicit, and sacred, assumption of Scottish Law.
Again, because if you truly believe no crime was committed, then it would be an offense against God to declare a man guilty of one.
...And again... I think this is a very useful concept.
Here we call it Jury Nullification, and it's generally frowned upon. In Scotland, it's just "the way it is and should be".
Unfortunately, the parliamentary commission reviewing Scottish law and practices is trying to get rid of these; to bring Scotland in line with law around the U.K. and the EU.
Actually, they've more or less officially got rid of the latter... but Scottish juries still regularly return "not guilty" verdicts, for charges that are fully proven, but which they believe are not crimes, or for which there is no "guilt".
After all, you can't be guilty of something that wasn't wrong in the first place can you.
Something I think our own courts might be better off taking into account...