BE - As I sated, I suspect that the police and forensic team did such a poor job that all evidence had to be treated with a level of doubt and suspicion, even though some of it was probably very valid, take this point (from the Telegraph):

The bra clasp was only found six weeks after the initial crime scene investigation, by which time it had been kicked around the floor of Miss Kercher’s bedroom, leading to a high risk of contamination

If this is the level of forensic/police work that is presented in court, no wonder they had cause to disregard/ignore anything else the police put forward.