Some mods are sensitive the difficulty slider and do more than just "double damage" on high levels. Ascension is the best-known example of this, but some of my "improved Foo" mods do it as well. For example, Improved Bodhi uses more of her Undead Powers at higher difficulty levels and Undead Solaufein won't get his second wind on easy levels.
In the Improved Irenicus mod, if you are brave enough to play on Impossible the shards summon extra low-level monsters. So the Dragon summons Wyverns, the Sword summons Mordies, the Eye summons Beholders, etc. This encourages you to finish things quickly. It also gives victors (like Dundee Slaytern) extra bragging rights.
This is explained in the "HINTS" file that comes with the tactics mod.