I suspect it is due to your Expedite buffers being too small - the WO is not scheduled for completion close enough to the ship date of the SO - therefore the MR-F figures the WO must be for something else and is trying to generate another WO to satisfy the SO.
 
MR-F needs several things to be accurate/reasonable to produce logical results:
1.  On Hand Inventory to be accurate
2.  BOM to be accurate
3.  Inventory Lead times to be accurate, or at least reasonable
4.  SO, WO and PO dates to be accurate
5.  Expedite buffers to be large enough to handle any mismatch

I've been thinking about what you said, "the WO is not scheduled for completion close enough to the ship date of the SO".  A good portion of our business involves weekly releases against blanket orders, where 10 - 12 weeks may be on order at any given time.  And because the stamping business is measured in truckload quantities, a work order quantity depends on how much of that items raw material made it on the inbound truck.
So, there is one WO completion date that fills maybe 5 - 6 of those SO ship dates.  They will never match.  Then along comes another months releases, so how much raw do I need to get on the next truck and when?
 
It seems that the mrp equation would be simpler if just sales orders were considered for requirements, excluding work orders that are there to help fulfill those requirements.
 
Would it be tough to provide that option?
 
Thanks,
 
Wayne Walkup
Sun Manufacturing, Inc.