YouTube and most other multimedia sites have a custom normalization and compression algorithm that makes sure the volume on any given video isn't incredibly louder or quieter than the others on the site. This feature was introduced a few years ago, during the "loudness war" and is greatly appreciated by many audiophiles because it means that songs won't be mastered for their loudness, but rather for their acoustics, thus making the final product sound much better. Long reply short, YouTube lowers the volume to normalize all of the videos on the site so a user doesn't play video a that is quiet, making them turn up their speakers, then play video b, with is louder, which would damage the speakers because of the loud audio from the video and having the speakers playing at a high output.
