Why ths discount and total tax a cross each other?

When I put The Sales Invoice Then I Select The Item for rs.500 and i give a discount for rs.10 .When i give the discount price that discount rate is also get On a tax Charge also…But I Am Not Give Any Tax Charge.I Want To know Whats Problem is Done here???

Can you explain showing screenshots?

this s a screen shot

Can you check in version 5. Also adding discount after submission does not make sense.