New YouTube protocols use "adaptive" streaming.  As YouTube determines that there is enough bandwidth to support a higher audio bitrate, it starts a new feed to your computer with that bitrate.  This can happen a few times as YouTube determines the best quality it can send to you.  We happen to catch each of these better streams in the process, resulting in the best available video for our customers.  Typically, the final video with largest file size will be the highest quality, and this can be confirmed by getting the properties of the file from Windows File Explorer.

Also Youtube has changed the way they are streaming their media and Replay Media Catcher 4 does not support the mp4 file. We did release Replay Media Catcher 5 that fixes this issue: