It is rather a shame that manufacturers actually give boot dimensions in litres as opposed to a minimum measurement of H x W x D of actual useable space (ie how big/small a cube can fit in).
As you say, it all depends on actal shape, slope of the roofline, angle of the tailgate and even the rake of the rear seats which can affect the useable space.
When manufacurers boast about how ‘big’ the boot of a specific car is, they rather gloss over how much actual useable space there really is.
Dave.