Exchange rate in SO & Invoice

Hi All,

In SO Header level Exchange rate is X(Picked from OB08-correctly)

In SO item level Exchange rate is X

In Invoice of same SO Header level Exchange rate is Y

In Invoice of same SO item level Exchange rate is X

How this Y at Header level has come?Is it edited manually? IF YES then where to check whether Exchange rate is changed manually or not?

Reg,

Antaa21