How does Advanced Pricing calculate on Adjustments indicated as New Base Price?

I am doing some testing on using a price override on certain items that have a specific price for a customer that is different from all other customers.  The customer with the specific base price also gets and addition 15% discount on the item based on specific item groups.  I noticed during the testing that when I set the basis code for the 15% discount to "0" I get a different calculated values then when I set the basis code to "1".

For example, My price override adjustment has the new base price flag checked and the adjustment is set as a price add (option 5) of $309.  When I do an additional adjustment discount of 15% with the basis code set to 0 (% net price), the calculated value comes out to be $262.65.  When I change the basis code to 1 (% base price) it calculates to $264.  In both cases it is using the price override value to do the calculation but it is calculating the result differently.  Any idea why that would be?