I think your reasoning hinges on a few assumptions that are not quite right. You are expecting to be able to set a precise state of charge percentage, and you are expecting that the car can figure out exactly the current state of charge of the battery. Those are not possible, not at the precision you're asking for. If you are interested, you can read all the literature around battery voltages and how that voltage fluctuates, depending on battery chemistry. You would see that temperature plays a role, and that good measures can only be achieved in an open circuit, that sort of thing.
To make things simple, and because I am not an actual expert, I will say this: the car estimates the current total capacity of the battery and the current SOC, and it's pretty good at it, but it might be off by a few percent. That estimate will be constantly adjusted. In your specific case, by supercharging and driving etc you're raising the battery temperature. At home, eventually the battery temperature goes back down and the car starts thinking that the battery cannot hold quite as much energy. Since the charge is done, with the current voltage and less potential capacity, the overall SOC looks higher.
Said another way: it's not a bug where the car decides to charge more than you asked for. Would I prefer that it be perfectly precise? Yes. Is it technically possible? No.