The legal system is not very deterministic. There are several points in the process where human judgement, with few explicit rules to constrain it, determines or significantly affects the outcome.
Moreover, in many cases this is by design (insofar as any part of the legal system is by design), and there has been significant resistance to any attempt to make the process more deterministic.
In tort law, in negligence law, in the law of self defense, and in several other aspects of law, the "Reasonable Person" standard governs the outcome, or at least is important. This says that, in general, actions are OK if they are actions that a reasonable person would have taken in the ci8rcumstances. If a reasonable person would think the force used necessity and proportionate to the situation, self-defense is justified. Otherwise not. If a reasonable person would have taken the same care as the defendant, s/he was not negligent. If a reasonable person would not perceive a demand to stay out, the action was not trespass. If a reasonable person would not see actions as deceptive, there is no fraud. If a reasonable person would not be confused by a similarity of to brands marks, there is no trademark infringement. There is no explicit guidance as to who or what a reasonable person is. Case law sometimes says to consider "a person on the street", "a person met on a bus", or an "ordinary sensible person". In effect, the test means whatever the given judge or jury thinks reasonable. And judges and juries differ significantly on such matters.
Probable Cause and Reasonable Suspicion
An arrest, or a search warrant must generally be justifies by probable cause. But what constitutes probable cause is generally a judgement call for the judge or magistrate asked to issue the warrant, or assessing a police officer's actions after the fact. That judgement can and does differ from case to case, and from judge to judge.
A Terry Stop:
is another name for stop and frisk; the name was generated from the U.S Supreme Court case Terry v. Ohio, https://www.law.cornell.edu/supremecourt/text/392/1 (1968). When a police officer has a reasonable suspicion that an individual is armed, engaged, or about to be engaged, in criminal conduct, the officer may briefly stop and detain an individual for a pat-down search of outer clothing.
But what suspicion is "reasonable"? This requires a judgment call by the police officer, and then a later judgement by a judge asked to determine the lawfulness of the police action. While prior cases (precedent) give some context to this, different people involved can and do come to different decisions.
A key US defense in cases of claimed copyright infringement is the doctrine of fair use. This is intentionally flexible, and must be separately evaluates based on the detailed facts of each case. 17 USC 107 specifies four factors that must be considered, but allows othger unspecified factors to be considered as well. The law does not specify the relative weight or importance of the four factors, and different courts have said on different occasions that one factor or another is the most vital. Most often factors 1 and 4 have been treated as the most important, but not always. The non-statutory factor of "transformativeness" is often treated as a key aspect of the decision. There are no clear-cut rules for what will or will not be held to be a fair use in any particular case.
Beyond a Reasonable Doubt
In criminal matters, a defendant (in the US and UK at least) is not supposed to be convicted unless the case is proved "beyond a reasonable doubt". But there is no particulate standard for what constitutes a "reasonable doubt". (There is that word "reasonable" again.) Some have suggested that6 this is about a 90% to 95% certainty. But judges have firmly rejected such mathematical standards. In some US cases juries have been told that this means that they must be as sure as they would need to be to act on a major personal decision, like buying a house or making a major investment. But this formulation is far from universal, and would not result in identical results from different juries even if it were.
In copyright cases a key test is whether an allegedly infringing work shows "substantial similarity" to the claimed source work. This standard is not well defined, and explicitly appeals to the perceptions of ordinary, non-expert people experiencing the contested work. Obviously this is not a uniform standard, and results on the same comparison will differ before different juries or judges.
Unreviewablity of Jury Findings
Tje Seventh Amendment to the US Constitution provides that:
no fact tried by a jury, shall be otherwise reexamined in any court of the United States, than according to the rules of the common law.
This provision, along with older common-law tradition, has led to the rule that an appellate court will not overturn or alter factual determinations made by a jury, unless there is no evidence supporting them, or no rational jury could have come to such a decision.
This rule prevents appellate courts from imposing uniform standards of fact-finding on juries. Jury verdicts are notoriously unpredictable. Experienced lawyers have said that even in the face of the strongest evidence, there is always a 5-10% chance of a jury deciding the other way.