You are starting with an assumption that the percentage of charge (SOC) is a precise value. It's not. There are two values in play: the total capacity of the battery in kWh, and the current energy in that battery, again in kWh. Precise total capacity is impossible to know by the car, it's just an estimate. Estimated total capacity is updated from time to time by taking voltage measures when the car is off. This will depend if you let the car sleep or not, and at what SOCs you let it sleep. If the car stays between 50-80% all the time, the car will have a hard time estimating as voltages in that range are very similar. There are whole threads about this on this forum, with very knowledgeable people explaining it better than me.
Precise current charge is similarly impossible, you only get an estimate. You take those two together and come up with a %SOC.
Add to this the fact that as the battery temperature changes, its voltage changes and you'll start to understand why you cannot expect SOC to stay exactly as you set it.
In my case, I charge to 75% and come back the day after to a 77 or 78% SOC. The car was unplugged so it's not because it charged more. And I can see the charging stopped at the exact 75%. That variation is what I'm talking about.
