It's all about the location of ballast, VCG and hull shape It's as simple as that. Ballast to disp ratio does not tell you enough to arrive at any stability conclusions. If you have a B/D of 50% and that ballast is all in your bilge you may have a very tender boat. If you have a B/D of 35% and that ballast is all at the bottom of a deep fin you may have a very stiff boat.
refer to my previous post about the findings about changing the depth of the ballast. i will have to look for the quote, on that, to post. i don't know if i saved the link for that, or not.
anyhow, the ballast/displacement ration, as in most such numbers and ratings, is not an absolute. it's a guide. there are exceptions. however, it does give a very good idea about the stiffness of a boat. if you have a low ballast to displacement ratio, it means that there is a greater percentage of weight that is near or above the water line. as you noted, placement of weight is very important. weight not below the water line is a liability.
for instance, the B/D ratio of the average dinghy is much lower than that of the average keel boat. and, as that suggests, the average dinghy is far more tender than the average keel boat.
having ballast deep in the water does effect a boat's resistance to being heeled, but not as much as it might be assumed and at certain degrees of heel more than others.
stiffness usually means initial stability. if you have two boats with 35% of their weight as ballast and one with 50% of it's weight as ballast and you compare these boats in similar situations, you will see what i mean.
let's say that one of the 35% boats and the 50% boat have a common shallow fin and one of the 35% boats has a deep fin with a bulb.
stiffness, being initial stability, applys to the first....say 10 to 20 degrees of heel. after that, you are talking about ultimate stability. does she settle in and resist heeling forces or does she just keep on going over?
admitttedly, your deep finned 35% boat will tend to resist the heeling force of the wind to a greater degree than the other two, as it heels farther. that's because it has a longer righting arm.
however, at small degrees of heel ( where one refers to stiffness ) it doesn't have a longer righting arm than either of the other two boats because their ballast will be right below the CB. in that situation, the 50% boat will have a greater resistance to heeling because it will have more of it's weight below the CB.
now all of that is assuming that all three boats are the same in all aspects. if you change something, it will change things. ratios, like that, are kind of relative. regardless of the B/D differences of these three boats, if the 35% boat with the shallower fin was very beamy and flat bottomed, it would be stiffer than either of the other boats. that's form stability. however, that boat would have much less ultimate stability and would tend to stay upside down if it capsized. that would even be the case if the beamy 35% boat had a deep keel. the form stability would make it stiffer but it would also make it easier to capsize, once it reached a certain degree of heel, even though it's long fin would give it a longer righting arm. of course, that has to do with the shift of the CB as a boat heels.
all such ratios and numbers are useful in comparing the qualities of different boats but are only useful within a certain framework. there are more elements of boat design that effect the performance of the boat than just the single element covered by one of these numbers. that always has to be kept in mind. however, for the purpose of my posts, the B/D ratio is completely satisfactory in accurately making my point and it is a good reference to help understand the capabilities of a boat.
there is a tendency, i notice, for people to try to isolate one element of design....keel shape, rig type, etc....and say that any boat with that element will behave the same as any other boat with that same element, regardless of the other elements of vessel design that may be present. that kind of thinking is very misleading. as with anything else in the world, a sailboat is a culmination of ALL of it's parts and not just defined by one part.