strange behavior - on vat calculation

hi,

leasing entry is revenue type , in main page we have seected the customer name lets say "abc" than we are moving to billing tab and entring the details for billling.

here if we select same custoemr name as main page (abc) than system is calculating VAT on entered amount but if we change customer name to lets say " xyz" than system is not calculating tax.

main customer iss : MOHAMED AHMED GAD ELEMARY

and change : Sumbar Meredov .

below is the screen shot to verify