Sorry, should have referenced a source-- I edited my comment to include the source for the 46 (which is the exact average for salaried workers in the US).
I don't understand it either. I negotiated my salary for 37.5 hours a week of work. I'm happy to work a bit more when the work is interesting, but I'm going to show up late or leave early the next few days to make up for it. Alternatively, you can pay me more.
That's the deal. My work/bosses are happy with it. My co-workers seem to begrudge that they're freely giving up extra time and I'm not. I keep suggesting they stop doing that...
I don't know that I would take their numbers as gospel but to be fair in the tech industry it's probably more accurate when I think of it.