What are the reasons why this analog to digital conversion is inaccurate?

What are the reasons why this analog to digital conversion is inaccurate?

Helpful? Please support me on Patreon: https://www.patreon.com/roelvandepaar

With thanks & praise to God, and with thanks to the many people who have made this project possible! | Content (except music & images) licensed under CC BY-SA https://meta.stackexchange.com/help/licensing | Music: https://www.bensound.com/licensing | Images: https://stocksnap.io/license & others | With thanks to user Roymunson (electronics.stackexchange.com/users/134897), user Ken Shirriff (electronics.stackexchange.com/users/51939), and the Stack Exchange Network (electronics.stackexchange.com/questions/278350). Trademarks are property of their respective owners. Disclaimer: All information is provided "AS IS" without warranty of any kind. You are responsible for your own actions. Please contact me if anything is amiss at Roel D.OT VandePaar A.T gmail.com