I am secretly convinced they have "eco" programs just to get the A+++ rating on energy efficiency, but that you're not really meant to use those modes.
That's not a secret, that's exactly why they do it.
I feel like I read that in the manual that comes with my dishwasher. It was worded a bit differently, but it essentially said to use auto mode to get cleaner dishes, not eco which saves money but doesn't clean as well.
