There seem to be inconsistencies in their testimonies and that's why they are being re-interviewed. Sounds like someone is lying.
There are often inconsistencies in eye witness stories -- I would argue more often than not. That doesn't necessarily mean that anyone is lying. People can be, and often are, mistaken about things, particularly when things happened and what precisely they saw. Innocent bystanders of crimes often describe the perpetrator or the sequence of events that transpired differently, for instance. The same is true for police officers when testifying about how they conducted an arrest, a search or an undercover operation. In fact, I not too infrequently see prosecutors argue to juries during closing arguments that the inconsistencies in their witnesses' stories is a sign of their reliability because if they were all precisely consistent, it would be an indication that their stories were rehearsed and/or fabricated.