The "Update" button for the billing address field is completely counterintuitive and bad user design. Customers do not expect to need to (and should not have to) hit an "Update" button for their billing address to take on an order. We've taken hundreds of calls from customers who don't understand why their billing address is incorrect on their order.
It would be much more intuitive if, when the checkbox for "My billing and shipping address are the same" is unchecked, address validation on the billing address ran when the customer clicked the "Place Order" button.
Conversion rates go down for every step you put between a customer and hitting the "Place Order" button. This counterintuitive "Update" button step undoubtedly has a negative effect on conversion rates and should be fixed.