I have never noticed this behavior. You are of course, working with an "very" old version of NX. My memory doesn't go back that far.
You are sure that those "old" parts are not a part of the Step file or the embedded assembly structure ?
Ronald van den Broek
Senior Application Engineer
Winterthur Gas & Diesel Ltd
NX9 / TC10.1.2
Building new PLM environment from Scratch using NX12 / TC11