The difference is that in the majority of cases the appropriate standards for collection, transport, storage, and testing are used. Control samples are taken and tested as well, and also controls done before testing both control and suspect samples. This means that contamination can be managed and accounted for correctly and so any claim of contamination needs to be shown.
In this case the ILE video plainly shows that they failed to take adequate steps to prevent contamination, and in fact in many cases used techniques that if anything encouraged it. We also know that their transport and storage practices were sloppy (the knife managed to find its way out of a sample bag and into a shoebox for goodness sake!) and despite repeated requests, not one valid control test has ever been produced (in fact the prosecution tried twice to introduce controls, once by merely claiming they had and the court had lost them, and the second time by giving the court false controls.)
When those doing the testing fail to follow the correct procedures to prevent contamination at every single step, how can they claim that none occurred?
ETA: In fact we know for a fact that there WAS some form of contamination on the bra clasp because there was unidentified DNA on it. In other words, even if we assume that RS's was there, then once we remove his and MK's, there is still other people's, possibly up to another 3 people's DNA on that clasp. Now unless the prosecution's case is that those other three people all touched MK's bra clasp as well, the only way for that DNA to get there is via some form of contamination, either at the scene or afterwards, there is no other plausible explanation, and if up to three other people's DNA was on the clasp innocently, then why could it not have been the same for that which was claimed to be RS's?