The "Trying to Try"[1] post linked to early in this article has an answer to that: The slightest effort suffices to convince ourselves that we have done our best.
Work smarter and harder.
Couldn't you, in your head, convince yourself that you are working "harder", simply by imagining that you could work less hard than you currently are? Couldn't you do that, regardless of you current level of work? So does "work harder" even mean anything?
[1] http://www.overcomingbias.com/2008/10/trying-to-try.html