scheduling : level by level

Hi all ,

I have FG contains SFG1 , SFG1 contains SFG2 .

I am doing backward scheduling .

after MRP , in MD04 the planned order dates and time r as follows :

For FG : date time

Order finish : 30.03.2008 17:30:00

Order start : 25.03.2008 12:50:00

and for SFG1 :is in BOm of FG and seperate routing

: date time

Order finish : 25.03.2008 17:30:00 --> "this time is my problem "

Order start : 20.03.2008 10:00:00

why the ORDER finish TIME of SFG1 is 17:30:00 why it is not Order start time of FG.

Pl explain . whether it has to be happen like same above or we can make it to our rquirement... .

Regards