It could be the gap is a known 'danger area', where carp are often targetted, so are particularly wary of rigs.
It could be that your 'slightly supple' boom is sticking up from the bottom, whereas a totally soft braid could confuse them.
It is possible that by going softer your hooklink doesn't stick up out of the silt compared to the coated braid near the knot.
(Before anyone says anything targetted is the correct British spelling, single t is an Americanism)