“most countries now have pretty strict sentencing guidelines”

That’s a vast, vast overstatement.

“You should be able to predict the outcome of a court case if you have all the facts available. That's what fairness means.”

Too much of a simplification. The role of a jury is to interpret the evidence, every jury is unique. Evidence is not an absolute, there are no “facts”. A judge can include/exclude evidence that would sway a jury one way or the other. Sentencing, even without guidelines, is the least variable part of the criminal justice system in the western world.