One other thing that occurs to me is that the cross sectional shape of fly lines/heads, and most mono running lines are round and not really deformable in normal use of spooling onto a reel, but that most backing braid is deformable (can be flattened somewhat).
This makes the measurement of the volume of line/head, running line & backing 'interesting'.
In any event, consider that you cannot fill any container completely (leaving absolutely no spaces at all) with ball bearings; there will always be some space between the non-touching parts of adjacent ball bearings! [even with infinitely small ball bearings].
Thus, you cannot actually fill a spool completely with a fly line/head, running line, but the backing will 'bed in' and flatten somewhat as you wind it on.
So all spool capacity will tell you is the maximum you may fill that spool with round-sectioned line, allowing for the spaces between each coil of line, however small that line is.
Mike