Why do some of my maps have reduced in quantity of items dropped?

I set up some towers around and the map in the middle of all tower have 20% of quantity items dropped, left one have 89% and right one have 52%, I even have maps where I got "reduced x% in quantity of items dropped"
is this a bug or what is happening with those maps?

Randomly some maps have special modifiers on them. I would be that one you looking at is 100 percent xp or 50 percent rarity but 50 percent reduced quantity.
Kind of a garbage system as most the time you do not even realize these maps are there till you do them. Because the atlas is annoying enough without mousing over every single map and making an attack plan.
Atlas needs to go. Or something. Tired of running trash maps to set up a few good ones. Then crafting 50 decent maps to maybe get a 16 or two that is not total trash.
Or getting a god roll modifier map just to find out it is crypt or Mire and that you will get less mobs/xp/loot in it than you would a trash map because the maps are so horrible.

Some maps are corrupted (indicated by a red icon). Corruption can have both positive and negative effects.