I think the reason they did it this way is to have individual parameters
which are metrically independent.  Shell width and shell length are
independent parameters.  A change in one does not affect a change in the
other.  However, spire height and shell length are not independent, because
spire height is a subset of shell length.  An increase in spire height
(independent factors remaining constant) therefore produces an increase in
shell length as well.
For two shells of a given length, an increase in shell width will increase
the width-length ratio by an identical factor.  Likewise, for two shells of
identical aperture length (which for a Conus is essentially overall length
minus spire height, or body whorl length), an increase in spire height will
increase the spire-aperture ratio by an identical factor.  For example, if
the spire height increases by 32% (aperture length remaining constant) then
the spire-aperture ratio also increases by 32%.  This is ideally how you
would like a ratio to work - if the numerator doubles, the ratio doubles.
However, using the ratio of spire height to overall length, the situation is
quite different, because a change in spire height (independent factors
remaining constant) contributes to overall shell length.  In other words,
increasing the numerator of the ratio also increases the denominator, though
by a lesser amount.  Therefore an increase of 32% in spire height would NOT
result in an increase of 32% in the ratio of spire height to overall length.
Well, that's my guess.  Whew!
Paul M.