This is not always true. I watch a lot of video on the internet, and the default volume differs per site. On thedailyshow.cc.com for example, the volume is about 60% and I've seen of a few sites where the default is about 85%. I would think that a volume of about 60 would be right, as this allows users to both increase or decrease the volume how they like it.
What I mean by this is NOT that you should lower the volume of your video by 40%, but rather keep the volume the same and just "call" it 60%. This way, most users won't need the volume controls, but when needed the can adjust it both ways. And why 60 and not 10 for example? 10 would mean that the user can only lower the volume in 10 stages, and with a slider (like youtube does for example) it's almost impossible to distinguish between 10 and 6 for example, even though 6 and 10 would be a huge difference then. And with 60 as a default, users can increase volume by about 70% which should be more than enough in most cases.
On the other hand you could argue that 100% would be better, because it's easily recognisable. For example, if someone lowers the volume from about 60 to 30 and then want to reset it, how do they know what the default value was? With 100% this would be easier, as you could just crank it al the way up.
Keep in mind though, that users can always change system volume. I barely use the volume of the video itself, because system volume is universal for all applications. The only time i DO need it, is when I'm playing music and a video, or multiple videos at the same time, for example 1 music video for entertainment on the side while I'm actually watching a howto or something, which barely happens at all.