Labor party, labour movement - why a different spelling?

In a recent blog post a question was posed regarding the spelling of "Labor" vs "Labour" in the name of the Australian Labor Party.

On 20 November 1918 the Queensland Labour Party changed its name to the Australian Labor Party - Queensland branch, adopting the same spelling of "Labor."

In view of this, and to overcome the potential for confusion, many historians and others use the spelling "Labor" to refer to the political party and "Labour" to describe the extra-parliamentary movement.