Here is a post that will help explain a bit about the stat recommendations in general:
For your specific case, the 12% vers corruption effect really does affect what items are optimal for you significantly. You can exclude that item from BiB if you want and see how the recommendations change dramatically.
A couple things worth knowing about our healing optimization:
1.) We base everything on simulations that we do with our healing simulator. The raid healing is based on an AoE-healing heavy fight with tons of healing to do - very similar to the Vectus fight in Uldir (scaled up to current content levels). Since we are the only people doing simulation, our recommendations tend to be different than what is on-trend. I can tell you from experience that our recommendations do actually work very well in-game.
2.) Our simulator takes into account everything - including the damage reduction from Versatility. This makes us give vers more value than other sources of theorycraft.
3.) Our calculations assume that you are pushing your mana pool to the limit in a very hard fight. If you are instead optimizing for peak burst AoE healing and not suing HW: Serenity on cooldown - it would make sense for you to exclude that trait from the optimization, which you can do with the “customize” options.
4.) Healing is not like DPS - there are many ways to gear that will all be successful. If nobody dies, your gear is good or your team doesn’t need you 
We provide you with a very good solution out of the box that will work. It is one of many and it is often not going to be “on-trend” because we don’t follow trends, we follow our mathematical models. We provide the “customize” section for this very reason - use that to change things up to your personal preference.