Anderson Darling/ Normality - Question about a specific correction

In the below mentioned website, the Anderson Darling test is explained.
Can anybody tell me why there is a correction in the calculation of the Z-value (see column L of sheet 2 in the embedded excel-sheet of the below mentioned website).

The P value is not calculated as i/n. But corrected and is now calculated as (i-0,3)/(n+0.4)
Can anybody give some substantiation of the used 0.3 and 0.4. The N is maximized at n=200.

https://www.spcforexcel.com/knowledg...-for-normality

Why don't you ask the question on that website at the bottom of the article? The author (Bill) still seems to be supporting it, judging by recent comments there.

Pete