It may actually be that his test papers got mixed up with another student. If not, then the processing speed may be a factor. Is wait time also in his IEP, or is the IEP only for gifted? The teacher's account and your DC's account are consistent with each other in suggesting that she assumed that a correct response would necessarily occur within a certain time frame. Since she brought this up herself, perhaps that would be a good connection point to mention to her that he likes to take his time thinking carefully before he answers, so waiting an extra beat or two can be very worthwhile with him. On his part, eventually it will be valuable for him to learn to advocate for processing time for himself, both by explicitly asking for more time to think, and implicitly, by learning some cue words that signal thinking, instead of sitting in silence. (E.g., repeating/restating all or portions of the question, "hm," "let me think about that," "give me a moment," touching his temple or chin to signal thinking.)
