Apple

If you click the “learn more” link under that setting in the next menu it goes to a page where you can learn more about that feature, which also explains why it charges more than 80% sometimes

https://support.apple.com/en-gb/HT210512

Per Apple: “With 80% Limit enabled, your iPhone will occasionally charge to 100 percent to maintain accurate battery state-of-charge estimates.”

https://support.apple.com/en-us/HT210512

"Mine works... most of the time. I know it says that "With 80% Limit enabled, your iPhone will occasionally charge to 100 percent to maintain accurate battery state-of-charge estimates." but it seems like it's charging to 100% too often. Maybe it's by design, like if you leave it plugged in too much after it stopped at 80%, it will go ahead and fully charge it?! For example, my phone charged to 100% Saturday and I didn't charged it again till yesterday when it stopped at 80%, but today it went again to 100%. 2x full charges in 5 days it seems like a bit too much imo." This is what I said yesterday in another related post. Today it went again to 100% so I am 100% convinced this is bugged, there is no way it's charging 3x times to 100% in 6 days by design. I am on 17.1 btw.