That's because they're doing speculation on facts - and can get them wrong.

That they call it "a speaking of the truth" doesn't means it's epistemologically true or that they think it is true and only true with no element of error (any judge will admit to that).

It just conveys their wish and effort for it to be true. Calling a verdict merely: "What we think is true" doesn't have the same ring to it, nor would be as respected by the public as the word of the law. So there's that.

But a verdict, at least the part that is based on a statement of facts (X killed Y), is absolutely either true or false (regardless of whatever we know it or not to be such -- only one set of events transpired in the real world, either X killed Y or he didn't).

Whether e.g. some moral judgement is neither true nor false - it depends on a system of beliefs and viewpoints, regardless of what somebody did in the world.

E.g. "you are a sinner because you wear a mini-skirt". Well, for those who don't believe in such a moral, no, you're not.