Amount deducted off my credit card is slightly more than what my invoice indicates, why is this?

What country are you in ? In the US you you need to add on your state's sales tax to the store's listed prices.

Or did you add or change your credit card details on your iTunes account when making the purchase ? If you have then each time that you do so a small temporary store holding charge may be applied to check that the card details are correct and valid and that it's registered to exactly the same name and address as on your iTunes account - it should disappear off your account within a few days or so.

Store holding charge : http://support.apple.com/kb/HT3702