customer no different in sales invoice header to sales invoice line proplem

Hi guys when i make invoice to sales order i found customer no different from sales invoice header to sales invoice line

why this happen .

Ex

I make sales order so-12

NO in sales invoice header is 12

AND

Document No is 12 but sell to customer field different from sales invoice line to sales invoice header in value and not same

How to avoid this from happening and what is the problem if possible

you can see this link and you will see customer no different from sales invoice line table to sales invoice header

as this link as following(clear explain to problem)

http://www.mediafire.com/download/lu9ggaceuejxxp2/111.bmp

thanks