Ok A quick lesson in swimbait manufacturing.
Plastisol shrinks as it cools. Anybody who has poured baits knows what I am talking about. Hudds are made in a two piece mold and they are injected. The Plastisol is under preasure and forced into the mold. They remain under presure untill the mold has been cooled. this is done to help reduce shrinkage. The plastisol expands when it is hot, so the hotter it is poured the more it shrinks. Now how does this apply to the Hudds? Well, its simple. The bait is under preasure so insted of getting a "dent" in the surface as it shrinks it creates a hollow spot usualy in the biggest part of the bait. Now since the lead/foam insert is in the front half of the bait the hollow spot should and does occure in the area just behind it or in the tail.
Why do some come out solid and some hollow? There are several variables that determine this and any slight variations will cause diferences in the baits. I have held a bunch and I have never felt a solid one. I dont think this will have any negative affect on the bait unless the plastisol is thin around the hollow spot. Than it could potentialy be a weak spot. But the baits that I have seen torn usualy have the tails missing.