I also get this issue as well when I export to Hex and Import back into the Poser, even when I don't make any changes in the Hex.  The resulting morph, when loaded using morph target, is very slight and unnoticeable when the morph dial is only moved 0-1.  But you can clearly see the entire body part move when the dial is moved drastically.  I think the problem lies when exporing out of the Poser, that the figure is not completely zeroed even though all the parameter dial says so.  (Trust me, I've checked every single parameter dials and manually entered zero on every single one just to be sure.)
Plus, this problem doesn't happen when I export using the option "As Morph Target", as in no world transformation.  So, the figure is either off center in default, even very slightly, or the position of the figure is altered when loaded from the library.  
Personally, it's not a big issue for me because I don't notice this as long as morph dial is moved within the reasonable range.  But it's one of those annoying thing that really bothers me because I don't want every vertices to be altered when exported to Hex.