The percentages a given scroll drop is exactly equal, however the reason why people get confused and think 1 is rarer, is because the outdoor gnolls will never drop scroll 1, and can only drop scrolls 2, 3 or 4 at equal percentages, but inside, 1 2 3 4 all drop equally. This means that if your counting an equal hunt of all the gnolls inside and out looking for these, then yes, 1 will show up less often.
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