でしょう used without any implification of uncertainty, or seeking confirmation. What does it mean in these cases?

Question

So I noticed over time, that でしょう is used a lot in general speech, even when you can’t really translate it to conjecture/probability, nor is it seeking confirmation, since the speaker is stating facts. From what I can gather, maybe it adds a nuance that to the listener, the information may not be as set in stone as for the speaker? So they don’t want to sound rough?

This is a line from steins;gate: A character is trying to tell the other, that a press conference was cancelled (which as far as this episode is concerned, from the speaker’s perspective is a FACT), but the listener that he is telling this to doesn’t believe this

中鉢博士の会見は中止になったっしょ

っしょ here is the colloquial form of でしょう. I wonder why the speaker chose to say でしょう instead of something like よ　or って to further indicate the meaning of “It was cancelled!”. Does he use っしょ because even though the press conference being cancelled is a fact, it is not as far as the listener goes, and he is trying to remind him of it, like “It was cancelled you know!”

Another example, this time from Darling in the Franxx anime. Two girls are talking about another girl, the first girl is commenting on the eating manners of the girl in question. To this, the other girl replies with the second line. The girls know that the girl in question isn’t really human as she carries the blood of a different species and has horns, but she does look human overall. So they know it as a fact that the girl is not a human and yet...

彼女は変わった人だね
人じゃないでしょう ほら…

We see a でしょう again. Even though this is a fact, and the second girl is trying to remind the first girl of the fact, she uses でしょう, not よ or other methods for convincing.

Perhaps she uses this as a kind of reminder, again kind of like a “you know”, and while this fact is clear to her, the first girl doesn’t think exactly like this (she didn’t really consider the fact whether or not the girl in question is a human), and so she doesn’t want to sound too aggressive?

Answer 1

In both examples, しょ(う) is working as a reminder. The speaker is thinking the listener already knows the fact but has temporarily forgotten it. It's an extension of the しょう's basic role of seeking confirmation. It sounds slightly accusatory to me (∼ "come on", "hey").

中鉢博士の会見は中止になったっしょ
Dr. Nakabachi's press conference has been cancelled, {hasn't it / huh / you know / don't you remember}?

Please look again at the past context. (But if I remember correctly, the timeline around this 会見 is very tricky, so it may be intentional if there is some mix-up in the conversation.)

Answer 2

I think naruto's answer is pretty good in this case, but I think it's also worth noting that でしょう (without か) quite often does not really express "uncertainty" as much as "opinion"/"belief", so it is sometimes used even when people are stating things they are actually very sure of, simply to make it clear that regardless of how sure they are, they are only expressing their opinion or understanding of a matter (which they still believe is correct), but not trying to present themselves as some sort of authority on the situation (e.g. "I'm pretty sure it was cancelled (unless you have some new information I wasn't aware of?)").

I would also note that in your second example, she is using the word 人, not 人間. That is, as I understand it, she is not actually saying "she's not human" (人間じゃない) (which would be an established fact), but rather she's saying more "she's not a person (like us)" (because she's not human). That's really a much more subjective opinion, not a provable fact, which I think is part of the reason for using でしょう there too.