When I fly long distance I always make the effort to go in the (plane) bathroom and freshen up, brush my hair and such before I get there because I feel I get less hassle from immigration when I'm looking fresh as opposed to when I'm a red-eyed mess.

Despite the fact it should be perfectly expected for people to look like shit after a long-haul flight.

As you say, It's just human bias and people are affected by their biases even in roles where they should try not to be.