I do agree, and have been reading up on all this cu ft requirements and ports and calculations - what I have learnt points to 2 things
1. It assumes a perfect world - i.e the space in the boot is the space in the boot dosen't matter if you need more - short of driving round with the boot open
2. The cu ft of the base box has to also be measured in respect of the acoustic properties of the car it is to be fitted to, so just because the box has the correct air volume it may still sound bad.
At this point I started reading the calculations on porting/venting sub boxes and my head exploded
So yes I agree and I do think it should be a bit bigger - especially given the fact he has now put a 12" Sub in! - but the car still needs a boot otherwise Mrs Buster will be removing parts of Mr Buster's anatomy that he would much rather keep
The get out of jail card appears to be padding - by using nylon or loft insulation as padding you can artificially increase the cu ft of the box and yes there is a calculation for it and if you search hard enough you can find the acoustic variations caused by different padding material OMG!
So we built a box that fitted and put "that much" padding in it and turned it on - Buster liked the sound so it stayed.