because you are adding a 9 inch extension. 21.5+9= 30.5 Then you see I added 9 to the length of external port? that shows the calculator that those extra 9 inches will not be inside the box therefore they will not be subtracted from the net volume.
EDIT: Your final tuning will be 30.95Hz if you add that 9" extension.
dont you mean the final tuning will be 31.34hz with 30.5 port length. and thanks i was worried that i would only be tuned to 34.95hz with the extra 9 inches