Comment by stavros

I don't think that's how it works. Otherwise, a level 3 engineer would be working 40 hours a week but 4 engineer would be working 60 hours a week, which isn't the case.

These additional things a senior does that a junior doesn't aren't "write more code", they're "coordinate with people outside your team more", "be more self-directed", "be more reliable", etc. Things which don't take more time, but which juniors don't do.