I'm sorry, I forgot the most important part, which is why I don't believe a word... it's because it's self-serving. Any time a witness is testifying, the job on cross-examination is to expose the bias or self-serving interest that the witness may have in offering their testimony. When arresting officers testify against a defendant, they are of course just backing up their prior actions. Pretty damn rare is the officer who will admit on the stand that they bungled XYZ component of the arrest, or that they didn't actually have probable cause, etc. Because it's their arrest, it's their own record on the line, they have too great a self-interest in supporting their own arrest, and therefore their testimony should be discounted.

When an IA officer testifies against one of their own fabricating evidence, I can be sure it is testimony that they are thoroughly unhappy about giving and not particularly self-serving. Therefore, more trustworthy. Somehow I doubt, for example, that the IA department operates under quotas and driven by officer-arrest stats.