This has several reasons:
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Expert rumours will get better exp/h because orange salamanders is a great rumour when you can go to Necropolis. Aco also has one extra task total, so this would equalize the number of tasks per Expert Guild Hunter. In addition, this gives more flexibility when choosing your Expert rumour master, because of the variety of high-level tasks they now assign.
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Expert rumours have now become best for pet hunting when the player has no Hunter pets (apparently even for xp/h according to wiki). Ideally, Expert rumours should never be better than Master rumours in anything, especially with a block system that can take a long time to adjust. It just makes no sense!