Quote Originally Posted by Strangesounds
From Tifa's point of view, Cloud was not there (as we don't find out that Cloud was really one of the grunts in the background until later in the game). The fact that Cloud was able to recite the incident accurately shut her up because she was presumably in shock or just stunned; questioning her own perception of the events - as she herself was seriously injured by Sephiroth at the reactor.
Quote Originally Posted by phiefer3
plus the fact that while she was injured and likely going in and out of consciousness, she might have gotten a glimpse of Cloud (without his mask) trying to help her. Given her condition at the time, her own memory of those events at the reactor probably aren't the clearest. So her own foggy memories, which likely include a glimpse of Cloud, probably help to inject a little bit of doubt and confusion (along with the fact that she'd be wondering how Cloud could have known so much about what happened if he wasn't there), and so she may have wondered if she was the one who was wrong about what happened.