No prior experience of the case, means they took a completely open unbiased approach to the evidence.
Not entirely sure this is the case. It does rather depend how they were approached.
"Hey complete stranger, here's some evidence in a case, can you say if there was murder"
"Hey, person I vaguely know, here's some evidence in a murder case, the defendant was convicted, can you say if there was murder"
"Hey, valued fellow professional I've known for a long time and get on well with, here's some evidence in a murder case, the defendant was convicted, the prosecution used my research but they misinterpreted it, can you say if there was murder"
All of those are people who have no prior experience of the case, but they will bring a different level of bias, even on an unconscious level. It is virtually impossible for a person to have absolutely no bias.