[Mp4-tech] [H.264] Why B slice needs switch first 2 entries?

[Song]

Dear Experts,
I am reading the H.264 spec. 
I found some description about initialization of RefPicList1 in B slice. But some description is very hard to understand.
The following is the description at page 109 of ITU-T Rec. H.264 (03/2005)
8.2.4.2.3 Initialisation process for reference picture lists for B slices in frames
¨C When the reference picture list RefPicList1 has more than one entry and RefPicList1 is identical to the reference
picture list RefPicList0, the first two entries RefPicList1[ 0 ] and RefPicList1[ 1 ] are switched.
Can anyone explain the reason or the major purpose of this process?
What is the benefit?
Thank you very much.
Best Regards.
Bill
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