Extension fields in Advanced Find Form

Hi, experts,

     I have four solutions in which I added one extension field to a standard BO.

     Let's say they are S1,S2,S3 and S4 respectively and all are of the same data type.

     I added each of them to the Advanced Find Form of the standard BO's OWL.(From Extensibility Explorer)

    The strange thing is when I enter some data in one extension field (just say S1), other extension fields (S2,S3,S4) are also filled with the   same data from S1.The deleting also have the same behavior.And when I delete one extension field(just say S2), the data in other extension fields (S1,S3,S4) are deleted too.

Why is this happening?

How can I control that?

Thanks in Advance.

Regards,

Fred.