Speech checks have a magic number. If you've got more skill than that, you'll always pass. If you've got less, you never will. That's not realistic. Sometimes a sure thing goes south, and sometimes you get away with things you never should, just because you're having a lucky day.
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When you make a speech check in Skyrim, it compares it to a global value that stores the minimum speech. In the vanilla game, these values are constant and one perk is the only thing to change them. This mod will reroll the skill check threshold every time you pass a persuade check or every 24 hours. The values are normally distributed, so they'll usually be pretty close to the vanilla value plus or minus a few points. But, on particularly lucky days, the speech check will be quite a bit easier than it would otherwise. The reverse is also true; sometimes, on bad days, you'll have to have a higher Speech skill to pass the same check. This mod is intended to complement Andrealphus' How Hard Is This Persuasion Check? and I recommend that if you use this mod, you also use that one. It's good to have a sense of how hard a skill check is, but I don't want to know the exact value that will work every time. That makes it much less interesting. The default ranges of possible skill checks are centered on the vanilla values, but with a large standard deviation to provide variability. The rolls are capped between 0 and 100. However, all of this is configurable in the MCM if you don't like my settings (which is fair, no judgment).
Andrealphus already has a mod (Tweak 10) like this. Unlike this mod, the rerolls on the skill checks in that mod are evenly distributed. This means you're more likely to roll especially low or especially high for skill checks with his version. I recommend checking his out if you don't like my implementation, or if you're curious what the difference is.