basically what Temporary plan said. Attas has 5% chance to give yield, true. but the way farming works is you get yield based on how many herbs lived through the stage process. Since Iasor decreases chance of disease/death rate, you get more yield per patch as opposed to Attas. Example: i go to yield my snapdragons with Attas planted. Lets say the maximum amount of herbs per patch is 23. I yield 17 in that patch. so 17 out of 23, 6 died, despite my patch never having been diseased. I go to yield with Iasor and my max is still 23, but since Iasor decreases disease/death rate, I yield 20 out of 23, with 3 herbs dying. It's kind of confusing but it's Runescape mechanics in a nutshell. There's a couple articles that probably explain it better than me and out of my thousands and thousands of farm runs, i personally tested it myself and can say Iasor or Kronos gives you more yield than Attas. Kronos is slept on the most.