Well, it's possible that the code simply didn't check that it would come out nonnegative. Negative stats on items don't show up.
He didn't look at the result after one scroll? (I suppose that's kind of normal, there isn't much to look at with NPC-buyable 100% scrolls)
He didn't look at the result after one scroll? (I suppose that's kind of normal, there isn't much to look at with NPC-buyable 100% scrolls)