I'm thinking condensation due to the film being stored below freezing and brought into a high humidity and warm atmosphere prior to exposure.
I can't be sure of this but dampness may change photographic sensitivity slightly, that may account for the difference in density on the neg.
Just a guess...
Mark
PS haven't read the thread so sorry if this have been suggested and discounted
I'm just wondering if I presoak how much the development time is adjusted. Can't find any sensible information on the net - adding 50% to development time (like the source I quoted suggests) seems a bit long to me.