Not bias - Truth.
Many years ago a wealthy couple up the hill asked me to come up and show them how to make omelets as good as the ones I made.
I set up in her kitchen and stepped through making an omelet. At each step the lady of the house said, "Yes, that's just the way I do it".
So I had an omelet made and started to fold it out onto the plate when she said, urgently, "No, we like our eggs better done than that!"
My reply, "Oh no you don't. If you did you'd not have me up here showing you how to make omelets".
That was the only difference between my wonderful omelets and her so-so omelets. She was overcooking the eggs - not a lot, but enough to ruin them.