If it were real life then you’d be able to get a good indication of what you should/shouldn’t say next based on how the person looks, is talking, what situation you’re in, etc. Ingame, you don’t have any of that, so the percentage is just a simple system to give an indication of all those factors.
However, you could make it more complex by having it limited in accuracy at low skill levels and get more accurate as your speech skill increases.