Thanks for the tip on scanning at 1X - it never occurred to me to scan that slowly. But it makes some sense, as that's the speed that a standard DVD will be played at.
The booktype thing is a bit weird though. As pinto2 points out, the only thing it should do is to change a few bytes near the beginning of the burn.
Could it just be inconsistent burns or variable media that's causing this, and not the booktype setting at all? (ie) of the 20 or so discs that you burned, did all of the ROM booktype ones scan back bad at 1X near the beginning of the burn and did all of the other ones scan well near the beginning of the burn?