Unedited memory. As in the concept that perception vs. reality, and that perception is reality to the involved officer at the time. Plus the limited span of view on the camera. Conducting the interview with the officer first, gives a better understanding on what the officer perceived and what their thought process was that led them to act accordingly. After that, the officer is shown the video with the investigators.
With or without BWC's, the odds of an OIS incident being caught on video is large and growing. In the last 3 years, our Homicide has investigated 16 OIS incidents involving our officers (in addition to a few involving smaller regional agency's officers). Roughly 80% of those incidents were captured on video of some form, be it cell phones or security cameras. It's the world we live in now.