It happens all the time! Look back at statistics. Usually the sweet spot is performing after 60-70% of the bands have performed.
Again, if a band performs at the beginning, more than likely they will not get high marks. That's because there is so much more competition to see (Well I can't rate them bad, because I don't know if there are worse bands. And I can't rate them high, because I'm not sure they are the best).
Now, IF, after all bands perform, the judges can go BACK and calibrate, then I'm with you that no bias is performed.
And come on now, we ALL have bias. To think that won't be the case, is ridiculous.
Point being, whoever goes behind the champ, will get hosed (not purposely).