This all goes back to electrical theory and the direction of current flow and  this has seen some changes over the years as physicist and electrical engineers learn more about what is really happening.  I think Benjamin Franklin is involved in this story too at the very beginning.

Here is a little info on "current flow" theories.  But as mentioned, it doesn't really matter as long as every thing is hooked up correctly for which ever convention you like best.

 http://istp.gsfc.nasa.gov/Education/woppos.html