I think it’s showing that it was never a problem of not enough time to devote to any particular murder, but a lack of evidence due to the competence of the murderer. Essentially the murderers were more competent than the police for a certain subset of murderers, so even though there are fewer total murders happening the percentage of them being carried out competently remains the same.

Police in general are actually alarmingly bad at solving crimes. The ones they do solve tend to be cases where they either get incredibly lucky, the criminal is incredibly stupid, or both. Most cases are solved because the criminal was caught in the act, caught on camera, or told someone else about their crime and that person turned them in. Winning the case against them might come down to the evidence collected, but that initial arrest usually isn’t because of evidence left at the scene.