I believe schnoor11 has the right idea here:
Here is his post:
I am confused about something. And maybe you can help me out here.
I thought that any challenge (wheter initiated by the coach or by the officials) had to conclusively overturn the ruling on the field, or the play would stand as called. The ruling on the field was a completed pass. The officials would have had to find 'conclusive evidence' that this was an incomplete pass. Instead, after reviewing the replays, he comes back and states that there is no evidence of a completed pass? There was no need to find evidence of a completed pass. That was the original call!! It is not the ref's job to find conclusive evidence to support the call, it is to find conclusive evidence to overturn it.
Please let me know if I am incorrect, because I am confused.