editing a customer's invoice on the back end

hello,

there seems to be a problem with the invoice setup with regards to shipping and vat. i needed to edit a customers shipping as I had refunded some - i changed the shipping to £1.69. when I go to print out the invoice, it has changed the shipping to £1.13 and it has not changed the VAT amount. I thought perhaps that the £1.13 would be the £1.69 less the VAT, but this is not the case (if it was it would be £1.41). I then tried changing the shipping to £1.79, and it has changed on the invoice to £1.23 (I put the shipping up by 10p, and the invoice goes up by 10p?) and still the VAT figure remains the same.
so i see 2 problems - firstly that the VAT figure is not changing when i edit the shipping, and secondly that I don't understand what is happening when i edit the shipping figure.
any ideas?
kind regards,
Tanya