Depends. It's most likely how YouTube processes your videos. Usually within the first day or two, it'll playback at lower resolution then slowly regain it. But I've noticed that for Arma, both Arma 2 and 3, there are issues with playback on YouTube. It's probably a bit rate issue. After exporting your final video and uploading, YouTube compresses the video a little more so some information may have been lost during that process.
I would just experiment different methods and find what is best. Hope this helps.