Why same value coming at bill-to and ship-to customer tax reg number filed when we have separate num

Summary: For a Customer i have provided "taxpayer identification number" at header level but for this customer there are two different sites(bill-to and ship-to) with different tax registration number, so i provided respective registration numbers at site level for these two different sites, but after invoice creation when we view invoice from manage transaction page there are two columns "bill-to customer tax registration number" and ship-to customer tax registration number" for these two columns same value from header field "taxpayer identification number" is populated instead of tax registration numbers which have been provided at site level.

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