Many chests have a set pool to choose a pool from. Consider the inn chests that reward Ishtar scrolls where instead of always choosing from that pool that there is a secondary roll that is a very low, variable amount chance that a skua will be chosen instead or in addition to the ish roll pool. So when hitting up your inn chests you are guaranteed a roll from the Ishtar pool but you also have a 1 - 5% chance to hit the roll for an extra skua scroll or getting an exceptional roll lets you get a skua scroll.

Skua scrolls are merely a placeholder the sky is the limit for secondary reward or exceptional pools.