I'm waiting a bit because it seems like it might be the case for a number of items, I guess these are the bg1 only items, it looks like the BG2 ankheg plate mail is an ankheg plate mail + 2 as referenced here (http://gibberlings3.net/forums/index.php?showtopic=15700&p=134288). It could be up to EET to merge both ankheg plate mails during the merging process.
It's all guesses for now though, i'll test a bit more to find other concerned items.