Bias is an integral part of the human decision making process. It is rare indeed for anyone to have complete information when decisions must be made. The origin of bias is cultural training and personal experience. The function of bias is help one to make appropriate decisions by assuming the most probable state of the unknown facts.
This is the way bias works. There is no such thing as “normal” bias unless you are talking about a bell curve with regression analysis, because each person has a different experience.
Given this strong decision making adaptation humans are left with a difficulty in separating bias from fact, mainly because bias is supposed to help fill in the missing data.
Science requires replicable results for the exact same reason bias does not. The goal is discover as much of the unknown data as possible before taking a WAG based on previous experience.
If you want to prove something, the standard is construct and experiment that produces the same results every time. Once you have that, change one variable at a time to test and verify.
Bias exists. It is a good thing. But it is NOT science and CERTAINLY NOT fact.