I think I often push work without realizing it, but I feel that often I'm done around 16.00 or before lunch.
If I know I have to be there anyways, I might as well goof off a little.
If no-one cared about the hours but I just had hard deliverables every day/week/month, I'd work much faster so I can go home earlier.
Depends on the project, too. Some I'm more interested in than others.