Well, actually, that second Nat's Mark is my first from Pindle - the other one came from the Frozen River and I think another character. The only thing I've really gotten repeated is the M'Avina's True Sight - so, actually, I'm actually pretty impressed with the spread that my Zon has gotten from Pindle - but I have been thinking about drop mechanics and I don't think a map seed really impacts boss drops (but rack drops are definitely impacted).
The way I understand it, the item type is selected first, and then the item quality. So once a Hydra Bow is selected, THEN your MF can kick in and increase the chance for Unique. I've probably gotten 8 or 9 Hydra Bows since I started running him, so I did some comparisons from the ATMA chances and it looks like it's about 1 WF for every 32 Hydra Bows at 385%MF (though that ratio is different for some other items, like an HoZ is selected for every 19 Gilded Shields, so there appears to be a dynamic I'm not aware of) - so that's actually a workable number that should keep me motivated.