You are being requested to redo the pullups because either your video was done incorrectly - could not see your whole body, etc. or because your form or technique was incorrect. They want to see (in a correctly shot video) you do the pullups using the correct technique. Your CFA will not be passing until you show you can do the pullups.
Now, a fair question is say you put down you 15 pullups. But there were a few where you kicked your feet or it was unclear if your chin was over the bar. If you put on your original CFA that you did 15 pullups but when you redo the video with perfect cadence form, you can only do 10 (which is passing) what does USMA do? Do you have to take the whole CFA over again or do they just give you credit for 10 pullups? I would imagine that this happens quite a bit.