I wouldn't say 9/10 of the ramparts are a dud. You're just eyeballing it and making an assumption out of that. As more are dud then are actually popping, you remember more no-pops then pops.
Ever since I started recording how many pop versus no-pop ramparts we get, I've noticed that on average, around 25 to 29% of them will pop at least one NM. But this varies per floor. 1F have higher chance to pop while 4F has the least. On average, 7-8 ramparts are duds.
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