Why do some students rush to do an assignment while other procrastinate?

I finally gave my class to chemical engineering students yesterday.  I gave it in a computer lab so I observed students doing other work on their computers while I lectured.  I stressed that databases like Scifinder Scholar and Scopus are very fast but can't evaluate the data retrieved.  The researcher must take the time to read the abstract to see if the paper answers their question. 

When I gave the assignment I suggested to the students that they should execute the search while they are in class since the training passwords were valid for just one day.  I said they should retrieve a relative large answer set and take them home and take the time to evaluate them.  5 of the 13 students completed the assignment and sent me the results that afternoon even though they had a week to complete it.  Some of my thoughts:

• The students found the assignment too easy
• They were just disinterested and wanted to get it over with ASAP
• Other students just procrastinate

When I grade the assignments I will see if there is any difference in the grades between the "eager beavers" and procrastinators.