Automatic Drive: How Unconscious Cognitive Biases Help Fire Our Motivation

So how did participants keep themselves motivated when there was much less work to do? You guessed it: they under-estimated the number of t-shirts in the box.
By under-estimating their progress when they were near the end of the task, highly motivated people are able to push themselves on harder towards the end when the temptation is to slack off.

When people are highly motivated to achieve a task, they over-estimate their progress at the beginning and under-estimate it at the end. This helps provide us with the psychological energy to keep us going through the task.

http://www.spring.org.uk/2012/07/automatic-drive-how-unconscious-cognitive-biases-help-fire-our-motivation.php