I'm shooting in the dark, but can this be explained by the fact that people don't know whether the officer has a body camera or not, but won't take the risk? tbh I don't know how big/noticeable they are,.
Don't you think it's more likely that the police involved - who all knew about and experienced wearing the body cams for a portion of their shifts - modified their overall behaviour, than assume that the unrelated public - who very likely don't know that police are trialling the cameras, were told when they were being recorded, and who probably haven't individually had multiple interactions with police before/during the trial (ie. to learn and modify their behaviour, even when they knew they were not being filmed by body cameras) - all managed to modify their behaviour, without conferring or conspiring, in line with the trial time?
Honestly, I have no clue about the proportion of police abuse vs. false claims. I assumed the latter was more important, but I'm probably wrong, now that I think about it.