Most likely the odd fixture in this higmast was replaced at some point, they didn't want to replace all 12 fixtures so when one of them died or whatever they just replaced the one.

You might be right! Because I remember that GE discontinued the HMAA fixtures, and I guess TXDOT didn't have a choice, but to slap on a Cooper Galleria fixture. What if I would of suggest is to match the same high mast, is to put a Holophane high mast fixture, to match the other eleven fixtures.

Cooper makes a highmast though, don't they? Or was that discontinued as well?

Well I have seen entire highmasts in Texas with the square cooper galleria fixtures before, there are plenty of entire highmast setups with those. And if I do recall correctly I have seen highmasts in the state with mixed GE's and Cooper Gallerias as well. So they probably just used a fixture they already had in their warehouse whenever the other one died.

@streetlight98, Cooper Lighting still makes the HMX high mast fixtures, but TXDOT doesn't use them anymore.

http://www.cooperindustries.com/content/public/en/lighting/products/roadway_lighting.html