BKENTD LINE field keep updating every update

SOLVED

Hi all,

I am doing customization on Bank Entry Screen using ISV module called Extender. But that is not the problem. 

I notice that, on the Bank entry grid, every time I add new line, the LINE value will auto decrement -1,-2,-3. if I remove -2, and add new line, the line value will become -1,-3,-4. then I save the entry and what actually inserted in table BKENTD is line value 1,2,3. If I update and enter new line in the middle of 1 and 2, the new line id is -4, and supposedly I will have the line list as 1, -4, 2, 3. but when update the Bank entry, the line value is updated as 1,2,3,4; according to the row sequence. So the LINE column is actually a primary key that keep updated every time. And this give me problem because I have the LINE and SEQUENCENO column in BKENTD as foreign key in my custom table.

Is it just a bad planning that BKENTD do not have fix primary key column? 

Thank you.