I know the Blu-Ray version of 300 is in the aspect ratio of 2:40:1, however I noticed one day when watching 300 on HBO HD that hte aspect ratio was different.

To put it simply, the black bars that are present on the Blu-Ray version were gone and more picture filled the screen. I did a comparison with both versions paused and the image for sure was not stretched out and indeed the HBO HD picture revealed more picture.

Anyone have a reason as to why this is case?

Yep. The HBO version was presented in "open matte." The movie was shot in Super35 if I'm not mistaken, which means that to get the 2.40:1 aspect ratio the top and bottom of the film frame are cut off. HBO chose to leave the top and bottom on, but that's not to say it's better that way. When a movie is composed for the 2.40:1 AR the frame just might not look right when being reformatted. What the director chose not to show can be just as important as what he chose to show.

I'm not sure if I'm making a whole lot of sense, but if you go to http://bluray.highdefdigest.com/tags/show/Joshua_Zyber and scroll down to the bottom where he talks about aspect ratios and open matte, you'll find Josh Zyber's excellent explanation with pictures to boot.

Hope this helps.