This is not related to the discrepancy shown by the calculator. This discrepancy shows you what the sensitivity/360 distance is with the calculated sensitivity is compared to the target sensitivity/360 distance, and is caused by the lack of decimal support by the game.
The inaccurate slider note means that when you set the slider to say 3.3 it might in reality be anything from for example 3.21 to 3.39. This is the case for a lot of games unfortunately, and if there's no config file option there's not much you can do about it.
This will make the actual sensitivity differ from the calculation, but usually not by a significant amount. Although this will vary depending on your sensitivity. If your target sensitivity is for instance 0.2, and the actual sensitivity is 0.29 due to the slider being inaccurate, you will get a significant difference since it's 1.45 times higher.