I think it's to test under the same conditions, as far as possible. To see if anything affects the outcome that wouldn't necessarily be obvious otherwise, such as how clothing might inter-react or change outcome, for instance on the jeans there was a scratch on the car where there was a metal stud, and IMO as regards other clothing for instance - a cap peak might hit somewhere, certain shoe soles might affect movement, a shirt sleeve length might receive damage etc. I believe that is how scientific tests are conducted by replicating all details as far as possible. Otherwise, you would get a defence lawyer saying 'but you weren't wearing the same jeans the victim was wearing, so how can you compare the two?'
I don't think he engaged in subliminal messaging. There is no evidence for him being unprofessional or biased. JMO