Tax Rate Variance

We have 1 PO with 2 lines. This is an Expense Type PO.No Tax is calculated on PO.

This 1 PO is matched to 2 different invoices (in same OU). Tax is calculated on both the AP invoices.

Invoice 1, PO Line 1 --> Tax Rate Variance is calculated. No issues here.

Invoice 2, PO Line 1 & 2 --> Tax Rate Variance is not calculated

How come TRV is calculated for Invoice 1 and not for Invoice 2 ??

Thanks,

Siva