border change when inserting rows or columns

In sheet1，I select the row 10 to insert a row；
In sheet2，I select the row 9 to insert a row；
cell border style changed differently。

In sheet1，I select the row 9 to set top border and bottom boder；
In sheet2，I select the row 9 and row 10 to set top boder；

Different operations cause different border changes.


What are the special border changing rules when inserting rows or columns in excel?

border.xlsx

Cells with top and/or bottom borders are typically meant for Total/summation. Put in some numbers in these cells should help you better in understanding the rules when inserting rows.