Confusion in understanding how data stored ?

firstly :
i want to enter anew sales order >> deliver >> invoice >> pay   for this order .
these are the tables used to store data about sales orders
( "sales_orders" , "sales_orders_details" , "debtor_trans" , "debtor_trans_details" , "audit_trial" , "comments" , "refs" , "stock_moves" ).
"sales_order"and "sales_order_details" tables stores all kinds of orders as "sales quotation" , "sales order" , "direct delivery" and "direct invoice"
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"debtor_trans" and "debtor_trans_details" tables stores all transaction about sales order like (delivery , invoice , payment).
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if i use "direct invoice" Form it will stores as normal sales order in "sales_order" and "sales_order_details" tables   // that's very good - no problem
but
see this case (i entered 3 direct invoices   "1" with a value $1000 , "2" with a value $2000 , "3" with a value $3000 )
in "debtor_trans"  table
i fount that there are two transactions (deliver with type "13"  and invoice with type "10") stored for each one of these three "direct invoices"
but the problem is when i added the third "direct invoice" with id "3" anew transaction added to first direct invoice has id "1" in this "debtor_trans" table with type "12" that means "payment" with the third value $3000 like this table

debtor_trans
-------------------------------------------------
trans_no      type      ov_amount      alloc     
-------------------------------------------------
1                 10         1000               1000
-------------------------------------------------
1                 12         3000               3000
-------------------------------------------------
1                 13         1000                0
===========================
2                 10         2000               2000
-------------------------------------------------
2                 13         2000               0
===========================
3                 10         3000               3000
-------------------------------------------------
3                 13         3000               0
===========================

what is the problem ????
why there is no payment stored for sales orders "1","2" and after adding the "3" the payment of it allocated to "1" sales order ???