Editing Sale delivery

Hi dear,
While editing a sale delivery , i notice "THIS DELIVERY" cell shows stock in hand on that date. I think this cell must show the previous entered quantity . I make this clear with example.

For example

22-03-2014 I have 100 units in stock of item name "ABC"

23-03-2014 i make a sale delivery of 100 unit of Item "ABC",

Latter i want to reduce some quantity , i go to customer transaction inquiry and find that delivery, click edit, then it show the me 0 on "THIS DELIVERY" cell, rather than 100.

Thanks,
PHPDV