Regarding STO and OBD

Hi,

I have created STOs and OBD's by using BAPI_PO_CREATE1 function module,In my system after creating STO by using BAPI function module automatically OBD wiil be created for that STO.Finally i am getting like this .

For Example

STO NO.                       OBD NO.

5500063017                    10019544

5500063018                    10019546

5500063019                    10019545

5500063020                    10019547

Now my question is,in the series STO are created sequentially,but for that STOs OBD are created Zig Zag(Not sequentially).I don't know why..

Can anyone tell me?If anyone not understand my question please ask me.

Thanks