Why: Because P2010 and P2012 are apparently not directly compatible. It's like, say, Windows XP and DOS 5. So you can't count on keeping the same result, especially when batch processing! I hope you'll find a way to undo that, and then use P2012 only when needed. There is no advantage in batch processing the whole 200GB of photos anyway, it's not like it's a different file format and will spare you space, or anything like that. Except maybe if you're happier with the auto settings of P2012, which you are not, apparently.
Should Adobe have done more effort to ensure consistency/compatibility? Since they seem to have re-done much of the algorithms from bottom up, I don't know how easy it would have been for them to make the output stay consistent and it's questionable whether it would be worth the effort.
