The previous appeal was not based around the minute particle of gun powder residue - it was targeted at some of the decisions the residing judge had made in allowing spectacularly flimsy evidence to be presented to the jury - it was an appeal on technical legal grounds. Not surprisingly the appeal judges stood up for their mate and rejected it.
Not the fault of the jury, they have to be guided by the experts. In this case, the legal expert, ie the judge, and the forensice expert did not guide the jury correctly.
As in the case where that woman was convicted over the cot deaths - the doctor, believed the probability of two related events occurring could be found by multiplying together the probabilities. The bloke might be a good doctor, but he's a shiite statistician - but the jury were guided by his "expertise" in one subject into believing his views on another subject, in which he was not an expert, and should not have been allowed to give an opinion, and they convicted.
In the dando case, the foreman of the jury has now stated that without the residue evidence they would not have convicted. That doesn't mean they acted incorrectly - forensic evidence is enough to convict if supported by other evidence. The other evidence though shouldn't have been allowed either. The jury were given wrong guidance through the trial, and the checks and balances required to counteract this eventuality, ie the defence barrister (whom you might term a "conspiracy theory nut job" as the prime thrust of the defence put forward was that the 'serbs did it') and the court of appeal, both failed in their job.