There could be a few reasons for this! Remember, if you changed your subscription level since your last renewal happened, this changes the price also!
For members in the Domestic US, the most common reason is that a coupon or other promotional discount was applied to one order and not the other. This may also occur with an international order.
For international members, one common reason is the exchange rate. Currency conversion is handled by your financial institution based on the current exchange rate between US dollars and the form of currency you paid in. Please note that all amounts listed on the ZampleBox site are in US dollars.