When I read your first post I was going to respond "I'm a consultant and while I have to log 40 hours in the time tracker, I don't always bill 40 hours to clients". Much like yourself, if my clients don't have 40 hours of work for me, not much I can do about that. But next week they might have 60, so it balances out.

I've never worked a job where I didn't have to track my time, though. Four different companies, employed as full time staff, I've always had to justify where my time went each week.