Good explanation. It must be that the F6B rear shock is a rising rate shock ... as the motion of the shock progresses through its stroke, it gets progressively firmer. With the spring preload at "SOFT", the static weight mentioned by Travelor is compressing that shock through the first half of its travel, which may be the softest portion of its travel, and once bump energy hits the shock we are experiencing the final 2" of its travel, which is damped very firm.
Most modern snowmobile shock absorbers are rising rate ... they are designed to absorb small "trail chatter" but become progressively firmer as the stroke of the shock increases towards the stop.
On another note, I see that there is some disagreement about whether the dial adjuster is a damping adjustment for the shock (moves a piston in the shock, reducing travel but increasing hydraulic pressure) or a spring preload which has nothing to do with the shock directly except to create a condition that increases its travel.
On a snowmobile, it is almost ALWAYS better to go up in spring size rather than dial the existing spring up to a stiffer setting. This is where it is different than a bike. Cranking up springs eats up shock travel on a snowmobile because of how the skid is a self-contained unit; one is actually pre-collapsing the skid by doing this and taking away an inch or two of travel, which is also the least damped portion of the shock travel, which results in a harsh small bump ride. By going up a size in springs, you maintain the full travel of the skid (again, self-contained and coupled) but buy yourself additional control of the motion of the skid as a unit by controlling the static weight better. This, is turn, allows the shock (s) to do their "thing".