FRIGTHENING !! The thin part between the threads, is it due to metal loss or intended design ?
The thin part is likely from crevice corrosion and is not intended.
Crevice corrosion occurs with stainless steel when water and a lack of oxygen are present. If the keel bolts leak they can rot out from crevice corrosion, in the keel stub, where you can't see them eroding. No oxygen and leaking bolts can cause this phenomenon.
Also, if an owner attempts to seal a leak by covering the bolts with an epoxy or sealant, with the good intent of saving them and stopping the leak, and they continue to leak, crevice corrosion can eat away the keel bolts, under the "protective coating", and out of site, until they disappear and whither away.
Keel bolts can leak from BOTH sides. Keeping a dry bilge will alert you to a leaking keel bolt, from the outside, and prevent crevice corrosion from the top side at least. The worst ting you can do to a known leaking keel bolt is to seal it off from the inside or torque it until it stops leaking into the boat. If it's leaking in from between the hull to keel joint you have only sealed in the water, that already leaked in, and have just exacerbated and made the potential for crevice corrosion worse..